# Load On Demand Overview

The Load-On-Demand feature of **RadTreeView** allows child nodes to be added on the fly as parent nodes are expanded.

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![RadTreeView Load On Demand Overview](https://www.telerik.com/products/aspnet-ajax/documentation/assets/bb6d669402ae5265f172a72bbdc38020/treeview_loadondemandoverview.png)

The **NodeExpand** event fires when the "+" icon is clicked. This event provides the opportunity to add nodes based on the identity of the clicked-on parent node. The **NodeExpand**event is fired based on the **ExpandMode**property of each node. **ExpandMode**values can be:

* **ServerSide**: This setting triggers a post back to fire the **NodeExpand** event. See the [Server-Side Load On Demand](https://www.telerik.com/products/aspnet-ajax/documentation/controls/treeview/load-on-demand/server-side-binding) topic for an example of using this setting.

* **ServerSideCallback**: This value triggers an asynchronous callback that fires the **NodeExpand** event. See the [Client-Side Load On Demand](https://www.telerik.com/products/aspnet-ajax/documentation/controls/treeview/load-on-demand/client-side-binding) topic for an example of using this setting.

* **WebService**: This value causes the web service specified in the **WebServiceSettings** property to be called. The web service has a specific signature that returns an array of **RadTreeNodeData** types that automatically populate the child nodes of the clicked-on parent node. See the [Web Service Load On Demand](https://www.telerik.com/products/aspnet-ajax/documentation/controls/treeview/load-on-demand/web-service-binding) topic for an example of using this setting.

* **ClientSide**: This value signals that data has already been retrieved from the server. Once a parent node has all its children loaded you can set **ExpandMode** to **ClientSide**.

For other properties that control load-on-demand appearance and behavior, see the [Load On Demand Properties](https://www.telerik.com/products/aspnet-ajax/documentation/controls/treeview/load-on-demand/properties) topic.

For additional information on the pros and cons of the load on demand modes please check this blog post: [Load On Demand Modes - why so many?](https://blogs.telerik.com/aspnet-ajax/posts/08-08-08/radtreeview-for-asp-net-ajax-load-on-demand-modes---why-so-many.aspx)