The problem of estimation of four commonly used measures of overlap for two normal densities with heterogeneous variances is considered and relations between them are studied. Overlap coefficients are used frequently to describe the degree of interspecific encounter or crowdedness of two species in their resource utilization. Variants of overlap coefficients have also been used to estimate the proportion of genetic deviates in segregating populations and to measure racial segregation. Relations between four measures of overlap are studied and approximate expressions for the bias and the variance of the estimates are presented. The invariance property and a method of statistical inference of these coefficients also are presented. Results of some simulation studies concerning the performance of expressions for the bias and the variance are given.