The unregulated growth that is characteristic of human malignant gliomas is accompanied by, and may result from, losses and/or gains of genetic material. Understanding the mechanisms that underlie how particular genetic aberrations cause dysfunctional growth will help elucidate the pathogenesis of this disease. Two techniques are proving useful in evaluating the clinical relevance of specific genetic aberrations in malignant gliomas. Fluorescence in situ hybridization (FISH) permits direct visualization of gains and losses of genetic material in single cells and quantitation of cellular subpopulations that have particular genetic aberrations. Comparative genomic hybridization can identify regions of genetic gain and loss in tumor DNA.