Recent revolutionary advances in genetics bring a renewed importance to the behavioral phenotypes of mental retardation syndromes. Although the so-called" new genetics" calls for improved research on syndromic behavior, this work has not been a priority in the larger mental retardation field. Further, the work has suffered from inconsistent definitions and methodologies. In this paper key properties of behavioral phenotypes were clarified, including within-syndrome variability and between-syndrome similarities and qualitative differences. Three strategies were offered that improve the traditional focus on easily observed syndromic traits: a psychiatric approach, psychometric methods, and syndrome-specific observations. The need to combine these approaches was discussed as were complications of the work due to developmental and environmental issues.