If statement in model builder

[NadeemQazi]

I wish to use if statement in model builder. i am using buffer tool. I wish to use if statement in deciding the input feature  parameter of the buffer tool.  i do nt know how to give the condition in the if statement in model builder. any help

[DaleHoneycutt]

There's a series of blog articles that describe, in detail, how to do if processing in modelbuilder:
http://blogs.esri.com/esri/arcgis/2011/06/06/modelbuilderifthenelse1/

Also see this help topic.  I'd probably start with this topic, then look at the blogs above:
http://resources.arcgis.com/en/help/main/10.1/index.html#/Using_If_Then_Else_logic_for_branching/002...