With the development of accurate, flexible, and reliable digital field instruments, electrical methods of geophysics now are limited mainly by a lack of interpretation capability. Forward solutions for one-dimensional (1-D) (layered) models have been available for many years; they consist of Fourier or Bessel integrals, which can be easily evaluated numerically. Inverse solutions for 1-D models also have been used routinely for many years. However, calculating the response of a two-dimensional (2-D) or three-dimensional (3-D) model is much more difficult, except for certain simple geometries such as spheres or cylinders, where conductivity boundaries correspond to constant-coordinate surfaces. Advances in understanding and interpretation for electrical methods now depend on forward and inverse numerical solutions for arbitrary 2-D and 3-D models.