VSTO (Visual Studio Tools for Office) allows developers to customize Microsoft Office applications like Excel, Word, and Outlook using .NET languages. The latest version of VSTO is 3.0, which is included in Visual Studio 2008. VSTO allows developers to build add-ins that extend the functionality of Office applications. It provides features for workflow and SharePoint integration as well as support for Office 2007 applications. VSTO uses a manifest-based deployment model and requires the .NET Framework and VSTO runtime to be installed on client machines. Security is managed through .NET code access security.