Transforming a point: if you're talking about using matrices to manipulate points, I'm far from the best person to answer - have a look at http://www.gamasutra.com/features/19980703/quaternions_01.htm , which explains the maths behind quaternions in quite a friendly way (I also used this page extensively when writing the functions originally - many of the functions are basically exactly the same). The QuatToMatrix function there (listing 2) might be what you're after.