The introduction of new nonlinear optical crystals has provided researchers with a wide choice of materials for their particular applications. These materials potentially allow nonlinear interactions from the UV to the far IR. However, choosing the most appropriate material for a specific application is often difficult. For certain applications, traditional materials may have properties (other than the nonlinear coefficient) which make them very competitive with new, highly nonlinear materials. This choice is complicated further if additional crystal development is required. This paper will discuss, from the user's perspective, the material characteristics of KDP, KD*P, d-LAP, d-urea, BBO, AgGaS2, AgGaSe2, ZnGeP2 and others. The physical, optical and frequency conversion properties are summarized, with particular emphasis on the interaction of these properties. To provide a guide or tutorial to the user, several application analysis techniques are described and applied to specific examples. These include the calculation of first and second order phase matching characteristics and one and two dimensional SHG modeling.