Many methods of measuring niche overlap have been proposed by several authors, e.g. \textit{O. Renkonen} [Ann. Zool. Soc. Zool. Bot. Fenn Venano 6, 1-231 (1938)], \textit{M. Morisita} [Mem. Fac. Kyushu, Univ. Ser. E 3, 65-80 (1959)] and \textit{H. Horn} [Am. Nat. 100, 419-424 (1966)])]. In this paper, the authors propose conditions which overlap indices ought to have and give a general description of a good index which includes as special cases those of the well-known indices that satisfy all the conditions. The problem of estimating overlap from sample data is also discussed.