The electrical properties of a cell are maintained by ions moving into and out of the cell. This ionic movement produces electrical potentials, which regulate cellular excitability. The purpose of electrophysiology is to measure cellular excitability by looking at ionic flow and potentials across the cell membrane. This chapter discusses the interpretation of electrophysiological measurements taking into account two forms of in vitro electrophysiology: current clamp and voltage clamp. The chapter begins by looking at field potentials, which are measured extracellularly in the current clamp configuration. Special attention should be paid to the direction of potentials (sinks and sources), which are dependent on positioning of the recording electrode as well as the type of ions moving into or out of a cell. We then discuss field potentials at an axon, a synapse, and the types of fields typically observed. We finish the chapter discussing interpretations of voltage clamp recordings in which currents can be measured.