The electrical properties of the near-surface Earth depend on the chemical properties of the fluids filling pores, grain size, the geometry of the interconnected pore network and mineralogy of the solids. We first describe how electrical resistivity depends on the ionic composition of an electrolyte. We next discuss the controls on the resistivity of a porous medium. We start with Archie’s laws and summarize the development of the parallel conduction model used to incorporate surface conduction at the solid–fluid interface. We describe how the induced polarization (IP) effect in the case of a non-electronically conducting matrix is incorporated into the parallel conduction model through a complex surface conductivity. Models to describe the frequency dependence of resistivity in terms of a distribution of polarization length-scales, e.g. grain sizes or pore sizes, are reviewed. We discuss models to describe the mechanisms causing the large polarization enhancement observed in the presence of electronically conducting minerals and show how IP parameters are, in this case, related to the volume and size of the electronically conducting particles. We finish by considering the role of contaminants in modifying electrical properties of near surface materials and briefly consider the possibility of non-linear effects in measurements.