We utilize prototyping during all phases of each project to ensure the development is as efficient as possible.  In the beginning stages of development, this typically means mock-ups, form study models, electronic hardware simulations and virtual prototyping (FEA/CFD).  Once the design direction is verified and refined in these early stages, we move into appearance models, breadboards, and POC (proof of concept) prototyping and eventually into works like/looks like and pre-production prototypes.  We typically prototype in quantities ranging from 1-2 up to several dozen or even a few hundred for user testing and pre-production runs.