Transparent walls in buildings

I have been working on an interactive walk through of an Arena and when I move the 1st person camera into the build, many of the walls and doors are transparent. I am importing the models through FBX and they are built in Maya 2011. Is this a shader problem or a problem with the models themselves?

Thanks, Rob

You probably have to flip your normals in Maya. In game engines, you can only see polygons from one direction. Try turning on backface culling in Maya, so you will be able to see how it's going to look in Unity.

This page explains what normals are.