Until recently, women's institutions and their inmates have received little attention in the literature on prisons. This neglect stems partly from the fact that over time, women have comprised only a small fraction of the total prisoner population. Yet it is also the product of two common assumptions: that the development of the women's prison system and female experience of incarceration are irrelevant to mainstream penology just because they can illuminate the nature of the prison system as a whole. Neither assumption is correct. During the first stage in the development of the women's prison system (1790-1870), female penal units outwardly resembled male penitentiaries, but in some respects their inmates received inferior care. During the second stage (1870-1935), strenuous and often successful efforts were made to establish an entirely new type of prison, the women's reformatory, in which women would receive care more appropriate to their 'feminine' nature. Yet by institutionalizing differential treatment, the reformatories legitimated a tradition of providing care that, from a current perspective, was inherently unequal. In the third stage (1935 to present), the women's prison system continued to evolve in ways which perpetuated the older traditions of differential treatment. A total of 27 footnotes and about 130 references are provided. (Author abstract modified)