In this review, we detail the application of an AC calorimetry technique as it has been employed and optimized for the study of phase transitions, particularly the superfluid-to-normal transition and those undergone by liquid crystals systems while confined to geometries more restrictive than bulk. The theory of operation, the sample preparation, and the electronic equipment involved in such measurements are described in detail. Examples of its use in heat capacity measurements of two-dimensional helium films at sub-Kelvin temperatures and in liquid crystal studies, a discussion on the simultaneously measured phase shift, which contains information regarding the order of the phase transition, is also included.