In addition to their induced expression under various stress conditions, small heat shock proteins (sHsps) are also expressed during normal development in a wide array of organisms. Members of the sHsp family of Drosophila melanogaster are individually expressed at many stages of development, and their developmental expression contrasts sharply with their stress-induced expression. First, the developmental expression of sHsps is uncoordinated and each of the sHsps shows its own pattern of expression. Secondly this expression is highly regulated in a tissue-, cell lineage- and/or developmental stage-specific manner. An example of such regulation is during male gametogenesis when Hsp23, Hsp26 and Hsp27 are expressed in the absence of stress. However the expression of Hsp23 is restricted to cells of the somatic lineage while Hsp27 is mainly expressed in germ line cells and in some somatic cells. Heat shock does not alter the level nor the specificity of expression of these sHsps in this tissue while other HSPs, i.e., Hsp22 and Hsp70, are induced through signaling via the unique heat shock transcription factor (HSF). The HSF is shown to be present at a much lower level in testes than in other tissues and shows cell-specific distribution. The highly regulated expression of sHsps during development and differentiation is dependent on transcription factors other than the HSF. The specific expression patterns of individual sHsps suggest that these proteins may fulfill distinct functions during normal development. A search for partners of sHsps suggests that these proteins may function in cell proteolytic processes.