The systems of heavy quarks are particularly interesting because they lend themselves to effective field theory techniques. Typically this involves considering an expansion about the limit of infinitely heavy quarks. In this limit, the phenomenology of heavy quark systems differs qualitatively from light quark systems; this provides a window into the workings of QCD. However, in the real world, heavy quarks have a finite mass. This dissertation will examine a number of heavy quark systems and describe the associated phenomenology It will also probe the extent to which realistic systems are well approximated by expansions about the heavy quark limit. This will be done with direct comparison with experimental data and models with a finite heavy quark. In the end, this study will show that, many of the systems considered here, it is unlikely that realistic heavy quarks can be accurately described by such expansions.