Mental retardation(MR) and developmental delay(DD) are common disorders of departments of developmental and behavioral pediatrics. Chromosomal abnormalities, copy number variations, and gene variants are known causes of MR/DD. Genetic test has become the first-tier diagnostic method for MR/DD, with the widely used chromosome micro-array analysis and exome sequencing. Here we give a brief introduction of clinical approach to MR/DD patients, discuss some potential risks of genetic tests and propose some recommendations.