Restriction fragment length polymorphism (RFLP) analysis in the broad sense is the analysis of differences in restriction fragment pattern produced by defined target segments within or between cell compartments, cell types, etc., in a single individual or in different individuals. Thus both molecular hybridization and DNA amplification by two-primer extension using the polymerase chain reaction can define target segments for RFLP analysis. The two techniques are outlined with special consideration of characteristics important for genetic analysis. The mode of inheritance of restriction fragment patterns as a prerequisite for their use as genetic markers in inheritance studies is explained, leading to criticism of common usage. The importance of internal restriction sites for the determination of allelic variation is stressed. It is shown that, if target segments are under the control of a single nuclear diploid restriction fragment locus, then complete reconstruction of all parental target segments requires controlled crosses between individuals of like restriction fragment pattern.