Research indicates that there are physical and social, and possibly cognitive, differences between left-handers and right-handers. The three substantive sections of this colloquium paper cover brain functions, theories pertaining to the genesis of handedness, and cognitive development as related to handedness. Section 1 provides a brief description of the brain and reviews the concept of left and right brain. Section 2, on the genesis of handedness, reviews theories on birth position, trait inheritance, cultural influence, birth stress, pathological factors, neurological anomalies, familial learning, and genetic determinants. Section 3 concludes that differences in cognitive abilities of left- and right-handers are very small and of little practical importance. Sections 4 and 5 provide charts of difficulties many left-handed people face in their daily lives, and a summary of the relationship between the various research findings and handedness. Section 6 provides a partial listing of famous left-handed people. Section 7 cites approximately 15 references. (RH)