This chapter presents calculations of the frequency-dependent mode density in several different model microcavity structures. The calculations are performed using the “mode counting” method, which offers a simple and intuitive prescription for calculating the mode density in the cavities of many different shapes and dimensionalities. The presence of the cavity alters the mode density of the electromagnetic field coupled to the atom, thereby changing the atomic transition rate as calculated via the Fermi’s golden rule. Since mode counting is first and foremost meant to be a simple and intuitive computational prescription, unwanted analytical complexity is to be avoided, particularly if it hinders a deeper understanding of the physics of the mode density in one particular cavity or another. The effective mode density in this case is a series of the delta functions at frequencies corresponding to the resonances of the box.