This paper reviews a theory of pattern establishment and pattern restoration by endogenous ionic currents. These currents are supposed to be generated by a certain separation of ion leaks and ion pumps in cell membranes. In so far as these currents act back to further this separation, they would be part of a regenerative process that initially establishes positional values. Later in development, particularly in epimorphic regeneration, when positional values are restored or extended, these currents are supposed to leak through sites of discontinuity in such values and thus trigger growth. This paper also reviews the factual evidence for this view: evidence that developmental currents are, indeed, very widespread; evidence in a few cases, particularly in Cecropia follicles and in wounded cavy skin, that they can generate substantial voltage differences or gradients; evidence that comparable artificial fields can move charged macromolecules along cell membranes and polarize cell growth; and direct evidence in a few cases, particularly fucoid eggs, Cecropia follicles and regenerating amphibian limbs, that ion currents do, in fact, act back to direct or further development. The paper also presents a particular theory, based upon ionic currents, of the reversal of thyroid cell polarity by serum.