Within the last decade linkage analysis has become one of the most useful tools in the human genetics arsenal and is beginning to make substantial contributions in all areas of medicine. Its increasing popularity is a direct result of the burgeoning number of polymorphic markers that are now mapped to specific chromosomal locations. Within the near future the entire human genome is likely to be saturated. This paper is intended as an introduction to linkage analysis for non-geneticists. Basic methodological approaches, along with their strengths, weaknesses and assumptions are reviewed. A subsequent paper will critically review methodological difficulties in the application of linkage analysis to the psychiatric disorders.