diff --git a/src/vs/workbench/api/browser/mainThreadTreeViews.ts b/src/vs/workbench/api/browser/mainThreadTreeViews.ts
index 1df23985f9cef..faed925d70946 100644
--- a/src/vs/workbench/api/browser/mainThreadTreeViews.ts
+++ b/src/vs/workbench/api/browser/mainThreadTreeViews.ts
@@ -169,8 +169,8 @@ class TreeViewDragAndDropController implements ITreeViewDragAndDropController {
 	constructor(private readonly treeViewId: string,
 		private readonly _proxy: ExtHostTreeViewsShape) { }
 
-	async onDrop(dataTransfer: ITreeDataTransfer, targetTreeItem: ITreeItem): Promise<void> {
-		return this._proxy.$onDrop(this.treeViewId, await TreeDataTransferConverter.toTreeDataTransferDTO(dataTransfer), targetTreeItem.handle);
+	async onDrop(dataTransfer: ITreeDataTransfer, targetTreeItem: ITreeItem, sourceTreeId?: string, sourceTreeItemHandles?: string[]): Promise<void> {
+		return this._proxy.$onDrop(this.treeViewId, await TreeDataTransferConverter.toTreeDataTransferDTO(dataTransfer), targetTreeItem.handle, sourceTreeId, sourceTreeItemHandles);
 	}
 }
 
diff --git a/src/vs/workbench/api/common/extHost.protocol.ts b/src/vs/workbench/api/common/extHost.protocol.ts
index b8565df3724a7..f953182df5507 100644
--- a/src/vs/workbench/api/common/extHost.protocol.ts
+++ b/src/vs/workbench/api/common/extHost.protocol.ts
@@ -1271,7 +1271,7 @@ export interface ExtHostDocumentsAndEditorsShape {
 
 export interface ExtHostTreeViewsShape {
 	$getChildren(treeViewId: string, treeItemHandle?: string): Promise<ITreeItem[] | undefined>;
-	$onDrop(treeViewId: string, treeDataTransfer: TreeDataTransferDTO, newParentTreeItemHandle: string): Promise<void>;
+	$onDrop(destinationViewId: string, treeDataTransfer: TreeDataTransferDTO, newParentTreeItemHandle: string, sourceViewId?: string, sourceTreeItemHandles?: string[]): Promise<void>;
 	$setExpanded(treeViewId: string, treeItemHandle: string, expanded: boolean): void;
 	$setSelection(treeViewId: string, treeItemHandles: string[]): void;
 	$setVisible(treeViewId: string, visible: boolean): void;
diff --git a/src/vs/workbench/api/common/extHostTreeViews.ts b/src/vs/workbench/api/common/extHostTreeViews.ts
index e4730e17821fe..f5e63bdc8cc0c 100644
--- a/src/vs/workbench/api/common/extHostTreeViews.ts
+++ b/src/vs/workbench/api/common/extHostTreeViews.ts
@@ -10,7 +10,7 @@ import { URI } from 'vs/base/common/uri';
 import { Emitter, Event } from 'vs/base/common/event';
 import { Disposable, DisposableStore, IDisposable } from 'vs/base/common/lifecycle';
 import { ExtHostTreeViewsShape, MainThreadTreeViewsShape } from './extHost.protocol';
-import { ITreeItem, TreeViewItemHandleArg, ITreeItemLabel, IRevealOptions, TREE_ITEM_DATA_TRANSFER_TYPE } from 'vs/workbench/common/views';
+import { ITreeItem, TreeViewItemHandleArg, ITreeItemLabel, IRevealOptions } from 'vs/workbench/common/views';
 import { ExtHostCommands, CommandsConverter } from 'vs/workbench/api/common/extHostCommands';
 import { asPromise } from 'vs/base/common/async';
 import { TreeItemCollapsibleState, ThemeIcon, MarkdownString as MarkdownStringType } from 'vs/workbench/api/common/extHostTypes';
@@ -129,22 +129,22 @@ export class ExtHostTreeViews implements ExtHostTreeViewsShape {
 		return treeView.getChildren(treeItemHandle);
 	}
 
-	async $onDrop(treeViewId: string, treeDataTransferDTO: TreeDataTransferDTO, newParentItemHandle: string): Promise<void> {
-		const treeView = this.treeViews.get(treeViewId);
+	async $onDrop(destinationViewId: string, treeDataTransferDTO: TreeDataTransferDTO, newParentItemHandle: string, sourceViewId?: string, sourceTreeItemHandles?: string[]): Promise<void> {
+		const treeView = this.treeViews.get(destinationViewId);
 		if (!treeView) {
-			return Promise.reject(new Error(localize('treeView.notRegistered', 'No tree view with id \'{0}\' registered.', treeViewId)));
+			return Promise.reject(new Error(localize('treeView.notRegistered', 'No tree view with id \'{0}\' registered.', destinationViewId)));
 		}
 
 		const treeDataTransfer = TreeDataTransferConverter.toITreeDataTransfer(treeDataTransferDTO);
-		if (treeDataTransfer.items.has(TREE_ITEM_DATA_TRANSFER_TYPE)) {
-			const sourceHandles: string[] = JSON.parse(await treeDataTransfer.items.get(TREE_ITEM_DATA_TRANSFER_TYPE)!.asString());
-			const sourceElements = sourceHandles.map(handle => treeView.getExtensionElement(handle)).filter(element => !!element);
-			if (sourceElements.length > 0) {
-				treeDataTransfer.items.set(TREE_ITEM_DATA_TRANSFER_TYPE, {
-					asString: async () => JSON.stringify(sourceElements)
-				});
-			} else {
-				treeDataTransfer.items.delete(TREE_ITEM_DATA_TRANSFER_TYPE);
+		if ((sourceViewId === destinationViewId) && sourceTreeItemHandles) {
+			const additionalTransferItems = await treeView.onWillDrop(sourceTreeItemHandles);
+			if (additionalTransferItems) {
+				for (const key of additionalTransferItems.items.keys()) {
+					const item = additionalTransferItems.items.get(key);
+					if (item) {
+						treeDataTransfer.items.set(key, item);
+					}
+				}
 			}
 		}
 		return treeView.onDrop(treeDataTransfer, newParentItemHandle);
@@ -404,6 +404,21 @@ class ExtHostTreeView<T> extends Disposable {
 		}
 	}
 
+	async onWillDrop(sourceTreeItemHandles: TreeItemHandle[]): Promise<vscode.TreeDataTransfer | undefined> {
+		const extensionTreeItems: T[] = [];
+		for (const sourceHandle of sourceTreeItemHandles) {
+			const extensionItem = this.getExtensionElement(sourceHandle);
+			if (extensionItem) {
+				extensionTreeItems.push(extensionItem);
+			}
+		}
+
+		if (!this.dndController?.onWillDrop || (extensionTreeItems.length === 0)) {
+			return;
+		}
+		return this.dndController.onWillDrop(extensionTreeItems);
+	}
+
 	async onDrop(treeDataTransfer: vscode.TreeDataTransfer, targetHandleOrNode: TreeItemHandle): Promise<void> {
 		const target = this.getExtensionElement(targetHandleOrNode);
 		if (!target) {
diff --git a/src/vs/workbench/browser/parts/views/treeView.ts b/src/vs/workbench/browser/parts/views/treeView.ts
index e84fe69e9a1ce..c40d64a1475e4 100644
--- a/src/vs/workbench/browser/parts/views/treeView.ts
+++ b/src/vs/workbench/browser/parts/views/treeView.ts
@@ -10,7 +10,7 @@ import { IKeybindingService } from 'vs/platform/keybinding/common/keybinding';
 import { IContextMenuService } from 'vs/platform/contextview/browser/contextView';
 import { MenuId, IMenuService, registerAction2, Action2 } from 'vs/platform/actions/common/actions';
 import { IContextKeyService, ContextKeyExpr, RawContextKey, IContextKey } from 'vs/platform/contextkey/common/contextkey';
-import { ITreeView, ITreeViewDescriptor, IViewsRegistry, Extensions, IViewDescriptorService, ITreeItem, TreeItemCollapsibleState, ITreeViewDataProvider, TreeViewItemHandleArg, ITreeItemLabel, ViewContainer, ViewContainerLocation, ResolvableTreeItem, ITreeViewDragAndDropController, ITreeDataTransfer, TREE_ITEM_DATA_TRANSFER_TYPE } from 'vs/workbench/common/views';
+import { ITreeView, ITreeViewDescriptor, IViewsRegistry, Extensions, IViewDescriptorService, ITreeItem, TreeItemCollapsibleState, ITreeViewDataProvider, TreeViewItemHandleArg, ITreeItemLabel, ViewContainer, ViewContainerLocation, ResolvableTreeItem, ITreeViewDragAndDropController, ITreeDataTransfer } from 'vs/workbench/common/views';
 import { IViewletViewOptions } from 'vs/workbench/browser/parts/views/viewsViewlet';
 import { IConfigurationService } from 'vs/platform/configuration/common/configuration';
 import { IThemeService, FileThemeIcon, FolderThemeIcon, registerThemingParticipant, ThemeIcon } from 'vs/platform/theme/common/themeService';
@@ -216,7 +216,7 @@ abstract class AbstractTreeView extends Disposable implements ITreeView {
 		this.collapseAllToggleContext = this.collapseAllToggleContextKey.bindTo(contextKeyService);
 		this.refreshContextKey = new RawContextKey<boolean>(`treeView.${this.id}.enableRefresh`, false, localize('treeView.enableRefresh', "Whether the tree view with id {0} enables refresh.", this.id));
 		this.refreshContext = this.refreshContextKey.bindTo(contextKeyService);
-		this.treeViewDnd = this.instantiationService.createInstance(CustomTreeViewDragAndDrop);
+		this.treeViewDnd = this.instantiationService.createInstance(CustomTreeViewDragAndDrop, this.id);
 
 		this._register(this.themeService.onDidFileIconThemeChange(() => this.doRefresh([this.root]) /** soft refresh **/));
 		this._register(this.themeService.onDidColorThemeChange(() => this.doRefresh([this.root]) /** soft refresh **/));
@@ -1215,8 +1215,16 @@ export class TreeView extends AbstractTreeView {
 	}
 }
 
+const TREE_DRAG_SOURCE_INFO_MIME_TYPE = 'tree/internalsourceinfo';
+interface TreeDragSourceInfo {
+	id: string,
+	itemHandles: string[];
+}
+
 export class CustomTreeViewDragAndDrop implements ITreeDragAndDrop<ITreeItem> {
-	constructor(@ILabelService private readonly labelService: ILabelService) { }
+	constructor(
+		private readonly treeId: string,
+		@ILabelService private readonly labelService: ILabelService) { }
 
 	private dndController: ITreeViewDragAndDropController | undefined;
 	set controller(controller: ITreeViewDragAndDropController | undefined) {
@@ -1225,8 +1233,13 @@ export class CustomTreeViewDragAndDrop implements ITreeDragAndDrop<ITreeItem> {
 
 	onDragStart(data: IDragAndDropData, originalEvent: DragEvent): void {
 		if (originalEvent.dataTransfer) {
-			originalEvent.dataTransfer.setData(TREE_ITEM_DATA_TRANSFER_TYPE,
-				JSON.stringify((data as ElementsDragAndDropData<ITreeItem, ITreeItem[]>).getData().map(treeItem => treeItem.handle)));
+			const treeItemsData = (data as ElementsDragAndDropData<ITreeItem, ITreeItem[]>).getData();
+			const sourceInfo: TreeDragSourceInfo = {
+				id: this.treeId,
+				itemHandles: treeItemsData.map(item => item.handle)
+			};
+			originalEvent.dataTransfer.setData(TREE_DRAG_SOURCE_INFO_MIME_TYPE,
+				JSON.stringify(sourceInfo));
 		}
 	}
 
@@ -1268,6 +1281,8 @@ export class CustomTreeViewDragAndDrop implements ITreeDragAndDrop<ITreeItem> {
 			}
 			return previous;
 		}, 0);
+
+		let treeSourceInfo: TreeDragSourceInfo | undefined;
 		await new Promise<void>(resolve => {
 			if (!originalEvent.dataTransfer || !this.dndController || !targetNode) {
 				return;
@@ -1276,9 +1291,13 @@ export class CustomTreeViewDragAndDrop implements ITreeDragAndDrop<ITreeItem> {
 				if (dataItem.kind === 'string') {
 					const type = dataItem.type;
 					dataItem.getAsString(dataValue => {
-						treeDataTransfer.items.set(type, {
-							asString: () => Promise.resolve(dataValue)
-						});
+						if (type === TREE_DRAG_SOURCE_INFO_MIME_TYPE) {
+							treeSourceInfo = JSON.parse(dataValue);
+						} else {
+							treeDataTransfer.items.set(type, {
+								asString: () => Promise.resolve(dataValue)
+							});
+						}
 						stringCount--;
 						if (stringCount === 0) {
 							resolve();
@@ -1287,6 +1306,6 @@ export class CustomTreeViewDragAndDrop implements ITreeDragAndDrop<ITreeItem> {
 				}
 			}
 		});
-		return this.dndController.onDrop(treeDataTransfer, targetNode);
+		return this.dndController.onDrop(treeDataTransfer, targetNode, treeSourceInfo?.id, treeSourceInfo?.itemHandles);
 	}
 }
diff --git a/src/vs/workbench/common/views.ts b/src/vs/workbench/common/views.ts
index d837af0438bce..7f72eedc88b2a 100644
--- a/src/vs/workbench/common/views.ts
+++ b/src/vs/workbench/common/views.ts
@@ -838,9 +838,8 @@ export interface ITreeViewDataProvider {
 	getChildren(element?: ITreeItem): Promise<ITreeItem[] | undefined>;
 }
 
-export const TREE_ITEM_DATA_TRANSFER_TYPE = 'text/treeitems';
 export interface ITreeViewDragAndDropController {
-	onDrop(elements: ITreeDataTransfer, target: ITreeItem): Promise<void>;
+	onDrop(elements: ITreeDataTransfer, target: ITreeItem, sourceTreeId?: string, sourceTreeItemHandles?: string[]): Promise<void>;
 }
 
 export interface IEditableData {
diff --git a/src/vscode-dts/vscode.proposed.treeViewDragAndDrop.d.ts b/src/vscode-dts/vscode.proposed.treeViewDragAndDrop.d.ts
index 70fd7dc8b5aa5..b2ae05a46a2a1 100644
--- a/src/vscode-dts/vscode.proposed.treeViewDragAndDrop.d.ts
+++ b/src/vscode-dts/vscode.proposed.treeViewDragAndDrop.d.ts
@@ -33,32 +33,28 @@ declare module 'vscode' {
 	export interface TreeDataTransfer {
 		/**
 		 * A map containing a mapping of the mime type of the corresponding data.
-		 * The type for tree elements is text/treeitem.
-		 * For example, you can reconstruct the your tree elements:
-		 * ```ts
-		 * JSON.parse(await (items.get('text/treeitem')!.asString()))
-		 * ```
+		 * Trees that support drag and drop can implement `DragAndDropController.onWillDrop` to add additional mime types
+		 * when the drop occurs on an item in the same tree.
 		 */
-		items: { get: (mimeType: string) => TreeDataTransferItem | undefined };
+		items: {
+			get: (mimeType: string) => TreeDataTransferItem | undefined
+			keys: () => IterableIterator<string>;
+		};
 	}
 
 	export interface DragAndDropController<T> extends Disposable {
 		readonly supportedTypes: string[];
 
 		/**
-		 * todo@API maybe
-		 *
 		 * When the user drops an item from this DragAndDropController on **another tree item** in **the same tree**,
 		 * `onWillDrop` will be called with the dropped tree item. This is the DragAndDropController's opportunity to
 		 * package the data from the dropped tree item into whatever format they want the target tree item to receive.
 		 *
 		 * The returned `TreeDataTransfer` will be merged with the original`TreeDataTransfer` for the operation.
 		 *
-		 * Note for implementation later: This means that the `text/treeItem` mime type will go away.
-		 *
 		 * @param source
 		 */
-		// onWillDrop?(source: T): Thenable<TreeDataTransfer>;
+		onWillDrop(source: T[]): Thenable<TreeDataTransfer>;
 
 		/**
 		 * Extensions should fire `TreeDataProvider.onDidChangeTreeData` for any elements that need to be refreshed.