DNA variations have contributed enormously to the fields of medical and forensic science, especially through their use in studies on genes responsible or susceptible to various diseases and those on screening of chromosomal abnormalities in tumors. The types of genetic variations used in these studies have changed in the past 25 years and can be classified into five major classes: RFLP (restriction fragment length polymorphism), VNTR (variable number of tandem repeat), STR (short tandem repeat or microsatellite), SNP (single-nucleotide polymorphism) and CNV (copy-number variation). Genetic linkage analysis using these tools helped to map and discover genes responsible for hundreds of hereditary diseases. Furthermore, construction of the international SNP database and recent development of high-throughput SNP typing platforms enabled us to perform genome-wide association studies, which have identified genes (or genetic variations) susceptible to common diseases or those associated with drug responses. Genome-wide sequencing of individual DNAs is gaining immense scope. Here, I summarize the history of polymorphic DNA markers and their contribution to the genetic analysis of both rare hereditary diseases and common diseases, as well as recent advances in pharmacogenetics, including our contribution to these areas.