Genetic dissection of complex traits involves a series of analyses of phenotypic and genotypic datasets in samples of families and/or unrelated individuals. In particular, both linkage analysis and association analysis depend critically on the quality of phenotype and genotype data. The focus of this chapter is on certain types of genotyping errors and how marker data can help resolve problems of mis-specification of familial relationships and misclassification of genotypes, and on how those kinds of errors can be detected and corrected. The impact of these types of errors on the results of genetic analyses will also be discussed. Other types of errors such as measurement error or misclassification in regard to phenotypes are not considered here.