Hermaphroditism is defi ned as the expression of both male and female reproductive function in a single individual either simultaneously or sequentially (Sadovy de Mitcheson and Liu, 2008). In fi shes three patterns of functional hermaphroditism have been recognized (Pandian, 2010): simultaneous, sequential and serial (Fig. 2). Simultaneous hermaphrodites function as male and female at the same time or within a short span of time. They do not undergo natural sex change. However, the sequential and serial hermaphrodites undergo natural sex change; the sequentials change sex only once in a single direction (protogyny: female to male; protandry: male to female) during their life time but the serials do it more than once in either direction. The ontogenetic pathways of sexual differentiation in these sex changing sequentials and serials are depicted in Fig. 38. Incidentally, it is known that a fraction of individuals in sex changing sequential populations does not undergo sex change: e.g., protogynics Pagrus pagrus, protandric, Diplodus vulgaris. Social pressure induced sequential (e.g., Cichlidae, Crenicara punctulata, Zupanc, 1985) and serial (e.g., Macropodus opercularis, see Pandian, 2011) sex changes do occur in a few gonochores. They are discussed elsewhere (Pandian et al., 2012).