This chapter discusses statistical methods to derive loss-of-heterozygosity (LOH) and allele specific copy numbers in cancer cells. SNP arrays enable simultaneous information about thousands of DNA markers and their allele specific copy numbers, but to extract the information advanced statistical tools are required. The chapter shows how Hidden Markov Models (HMM) can be applied to infer LOH from genotype data and how the allele intensities are informative about allelic copy number. The methods are illustrated by examples.