Summary There are three methods used in practice to detect human linkage. These are: (i)The sib-pair method of Penrose, applicable to pairs of brothers and sisters, when the parents’ genotypes are unknown or disregarded.(ii)The method of efficient scores (u-functions), due to Fisher and Finney, applicable to families consisting of 2 or more children, together with either or both genotypes of the parents if known.(iii)The method of likelihood ratio, or backward odds, due to Haldane & Smith, and most suitable for large pedigrees: this links up with maximum likelihood estimation. Each method has its own advantages and disadvantages. It is part of the purpose of this paper to present a connected account of the subject, showing the relations between the three methods. It appears that all are really different forms of the likelihood ratio test, suitably adapted for practical use. It is suggested that recessive children of cousin parents can be used to detect linkage. Methods of using sib-pair tables to calculate efficient scores are given. As the sib-pair table is convenient in use, methods are given to extend it to families of more than 2 sibs, though this may not be fully efficient. It is also pointed out that tables of lods (log odds) can be prepared for use with families of 2 or 3 sibs, and an example is provided.