Transdifferentiation may be generally defined as the change of one recognizable cell type to another different cell type. The term was first used by Selman and Kafatos (1974) to denote the change of the cuticular cells of the moth larval silk gland to those producing HCO3 during metamorphosis/development and has since been used in many different contexts. So as not to produce a welter of semantics to replace the term already in use, I shall instead categorize the phenomenon of transdifferentiation by levels: primary, secondary, and tertiary transdifferentiation. Primary (or true) transdifferentiation would include the cell-type conversion or cell metaplasia that is so well documented to occur in some amphibian eye tissues in vitro and in amphibian (newt) eye tissues in situ (Fig. 1). This level is characterized by verifiably postmitotic cells, terminally differentiated and producing a specific cell product, transforming into a completely different cell type with differing cell product(s). Secondary transdifferentiation is marked by the conversion of those cells or tissues not definitely demonstrable as terminally differentiated, i.e., from an embryonic or possible stem-cell source. Also included is the concept of transdetermination (Hadorn, 1965), in which certain groups of cells in Drosophila occasionally become determined or committed to a developmental fate different from that expected. Tertiary transdifferentiation would encompass other purported/ reported changes of tissue types, e.g., that of muscle to cartilage (Namenwirth, 1974), and of striated to smooth muscle in Anthomedusa as reported by Schmid and Alder (1984) and Weber et al (1987). The well-known plasticity of plant tissues, especially in vitro, is the rule, rather than the exception, and as a topic of transdifferentiation is beyond the scope of this chapter.