“Linkage” describes the situation in which two syntenic loci are inherited together. More specifically, two loci are said to be linked if they are close enough to each other on a chromosome that recombination during meiosis is uncommon enough for their cosegregation to be detectable within families. Thus, linkage is a property of loci . All linkage techniques are essentially designed to test for a statistical association between a marker (genetic or biochemical) and a phenotypic trait. Classical model-based (parametric) linkage analysis was developed to investigate the cosegregation of a genetic marker and a binary trait (generally, disease affection status) within pedigrees. Model-based linkage analysis of quantitative traits is also possible and forms the basis of this chapter. Methods based on the exact likelihood calculation are described in this chapter; Markov chain Monte Carlo methods are described in Chapter 6 .