Multiple myeloma (MM) is a clonal neoplasm of terminally differentiated plasma cells (PCs). Conventional cytogenetic analysis is one of the widely accepted DNA genome-screening tools for identification of chromosomal aberrations in MM. The success rate of detection of abnormal karyotypes by conventional cytogenetic analysis in myeloma is very low because of the low mitotic activity and content of PCs in bone marrow. The advent of comparative genomic hybridization (CGH) has opened a new possibility for characterizing chromosomal aberrations in myeloma cells. CGH is one of the powerful global assays for detecting changes in DNA copy number in a given genomic complement. This method involves competitive hybridization of differentially labeled test DNA (tumor) and reference DNA (normal) to normal human metaphase chromosome spreads. Based on the relative intensity of the two fluorescent colors, regions of a chromosome with gain or loss can be identified in a single hybridization. One of the advantages of CGH is that it can make use of archival materials when frozen tissues are not available, thus greatly expanding the number of cases that can be analyzed. Prospective and retrospective application of CGH to tumor specimens would permit clinical correlative studies of changes in DNA copy number with a much greater power than conventional cytogenetic analysis. The detailed procedures for CGH, including DNA labeling, hybridization, fluorescence microscopy, digital image analysis, and data interpretation, as well as the limitations of CGH are discussed in this chapter.