I have a ReportViewer control inside a VS2010 Visual WebPart deployed in SharePoint 2010. If I put the report inside an iFrame it works fine (just setting the src = the URL of the report). If I put it into a ReportViewer I get: The request failed with HTTP status 401: Unauthorized There is a script error as well: Webpage error details User Agent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; WOW64; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET4.0C; .NET CLR 3.5.30729; .NET CLR 3.0.30729; .NET4.0E) Timestamp: Thu, 25 Nov 2010 16:31:21 UTC Message: Sys.ArgumentNullException: Value cannot be null. Parameter name: panelsCreated[3] Line: 129 Char: 12 Code: 0 URI: http://sp1/ScriptResource.axd?d=-xH2htwpSITFBSwbFV7VDS5Q9mmVEgTgSvuk8Jnd0Aqfw1cDpd2fSDrxiYgThtNQaq_myOFBHF5amI3fSpKyzWC4_s-HZtrKUr2-4SPgMT-QTYS4o9bFW4nEbGrr672JSI6F-Bj9IwIMCeRG3jk4zDu2TszO2ivKeIwAwEZMMVRfONWH0&t=5c2f384e I have deployed the webpart to a standard web application and it works fine. Here's the entirety of the code (or the bit that matters) <% @ Register Assembly="Microsoft.ReportViewer.WebForms, Version=10.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" Namespace="Microsoft.Reporting.WebForms" TagPrefix="rsweb" %> < br /> < rsweb:ReportViewer ID="ReportViewer1" runat="server" ProcessingMode="Remote" Font-Names="Verdana" Font-Size="8pt" InteractiveDeviceInfos="(Collection)" WaitMessageFont-Names="Verdana" WaitMessageFont-Size="14pt" Width="855px" > <ServerReport ReportPath="/PMReport" ReportServerUrl="http://sql1/ReportServer" /> </ rsweb:ReportViewer> Anyone have any ideas? Thanks Edward