Majority of human congenital disorders include mental retardation as a common manifestation. It was observed that little is done for diagnostic work up of such patients as the condition is perceived as incurable. We would like to emphasize that naming the disorder is important for better patient management, genetic counselling, and risk assessment in future progeny. In order to bridge the gap an attempt was made using constitutional chromosome analysis aided with detailed phenotypic characterization to get a primary clue for the underlying condition. The web-based software POSSUM served as having a clinical expert in our team of post graduate student researchers. This book will also serve as a ready reference for a fresher in the field of cytogenetics with detailed description of methodology.