The present work is an attempt to provide a systematic treatment of genetic linkage in diploid heredity. Part A presents a general account of statistical methods which can be brought to bear on the problem. The primary emphasis is on the practical aspects of estimation. A large proportion, if not the majority, of mutant genes fail to match up to 'textbook' genes-with faultless segregation ratios and expression-yet, these are the materials with which the practical researcher has to cope. For this reason, it is important to know how to deal with the assortment of genes which may display significant deviations from expectation. Part B examines the