This chapter concentrates on channels and pumps, which both act in unison to mediate cell function. Many different approaches can be used to investigate functional properties of channels and pumps. The chapter focuses on the use of the patch‐clamp technique to investigate ion permeation in channels and pumps. It begins with a historical overview of the whole‐cell patch‐clamp technique. The chapter then focuses on descriptions of ion channel conductance, selectivity and kinetics, and practical approaches to accurately quantify these parameters as derived largely from our own experiences in this area. It summarizes the results from patch‐clamp experiments that have elucidated the structural features of the ion permeation pathway in selected examples of channels and pumps. Two examples are given to illustrate the effects of voltage corrections on the quantification of relative ion permeability ratios.