This paper deals with the histological and histochemical aspects of ovarian development in the Nile Tilapia (Oreochromis niloticus). A total number of 96 fish, aged from one day to six months were used in this study during the period from May to December 2010. Ovaries of Nile tilapia were located in the posterior-dorsal part of the celomic cavity, ventral to the kidneys and the swim bladder, to which it was attached by the mesovary originating from the peritoneum. Ovary is covered with very thin capsule of fibrous connective tissue, the tunica albuginea, which give rises to ovigerous lamellae. The microscopic analyses revealed five stages of ovarian development in addition to oogonia: phases I (chromatin nucleolus stage) and II (perinucleolar stage; corresponding to the immature stage of development), phases III (cortical alveoli) and IV (previtellogenic; corresponding to the maturing stage), and phase V (vitellogenic; corresponding to the mature stage) and ripening stage. Ovaries of mature females presented different phases of oocyte development, indicating the multiple spawning habit of this species. Such information would be useful for studies related to the reproductive biology of Nile Tilapia that is used for commercial aquaculture around the world.