The type or namespace name 'CrystalReportViewer' does not exist in the namespace 'CrystalDecisions.Web' (are you missing an assembly reference?)

1. Right-click on "References" and select "Add Reference..."
2. Go to "Extensions"
3. Find "CrystalDecisions.Web"

and add namespace.

missed few assemblies in web.config

    <add assembly="CrystalDecisions.ReportAppServer.ClientDoc, Version=10.5.3700.0, Culture=neutral, PublicKeyToken=692fbea5521e1304"/>    <add assembly="CrystalDecisions.Enterprise.Framework, Version=10.5.3700.0, Culture=neutral, PublicKeyToken=692fbea5521e1304"/><add assembly="CrystalDecisions.Web, Version=10.5.3700.0, Culture=neutral, PublicKeyToken=692FBEA5521E1304"/><add assembly="CrystalDecisions.Shared, Version=10.5.3700.0, Culture=neutral, PublicKeyToken=692FBEA5521E1304"/><add assembly="CrystalDecisions.ReportSource, Version=10.5.3700.0, Culture=neutral, PublicKeyToken=692FBEA5521E1304"/><add assembly="CrystalDecisions.CrystalReports.Engine, Version=10.5.3700.0, Culture=neutral, PublicKeyToken=692FBEA5521E1304"/>    <add assembly="CrystalDecisions.ReportAppServer.ClientDoc, Version=10.5.3700.0, Culture=neutral, PublicKeyToken=692fbea5521e1304"/>    <add assembly="CrystalDecisions.Enterprise.Framework, Version=10.5.3700.0, Culture=neutral, PublicKeyToken=692fbea5521e1304"/>    <add assembly="CrystalDecisions.Enterprise.InfoStore, Version=10.5.3700.0, Culture=neutral, PublicKeyToken=692fbea5521e1304"/></assemblies>