This paper reviews the evaluation of bioelectric source strength and source field relationships for excitable fibers. For the single fiber, quantitative expressions describing the source may be derived which are independent of the fields produced by the sources. Rigorous expressions describe the equivalent elemental sources as discs, while the approximate line source is frequently satisfactory under physiological conditions. For fiber bundles the source associated with each fiber cannot be evaluated by the approximate isolated fiber expressions. However, when the bundle can be approximated as a bidomain and if the activation is planar, then mathematical expressions can be obtained. The resulting field behaves as if its origin was an equivalent single fiber. When the bidomain simplification and equal anisotropy ratio approximation is made, the planar waveform assumption can be removed and the resultant source and field can be evaluated. However, the latter are no longer independent.