Impedance spectroscopy has been widely applied over the last decades to study electrochemical systems and solid-state devices. However, performing impedance spectroscopy on emerging photovoltaics presents new challenges related to the unusual material properties and complex device architectures. This review provides an introduction to impedance spectroscopy for researchers in photovoltaics and closely related fields. The review begins with a list of practical guidelines for performing measurements and analyzing data. After this, the mathematical basics are reviewed, and an introduction to circuit elements is given. This is followed by tips for collecting reliable data and reducing artifacts in the frequency spectra. The review then surveys common approaches in the field for analyzing data, including performing equivalent circuit modeling, analysis of capacitance-frequency spectra, and carrier mobility measurements. The underlying assumptions of each analysis approach, as well as the advantages, limitations, and potential pitfalls are discussed.