There is strong evidence that multistep tumorigenesis begins with the acquisition of somatic mutations which promote genomic instability. Genomic instability is an important malignant trait because genomic instability can generate the genetic diversity that is necessary for the transforming cell to acquire increasingly variable and aggressive tumor phenotypes. Genomic instability often manifests in the form of chromosomal instability (CIN) leading to the induction of aneuploidy, a phenomenon identified by high resolution molecular cytogenetic techniques. Fluorescent in situ hybridization (FISH) and Array Comparative Genomic Hybridization (aCGH) are two high resolution molecular cytogenetic techniques that allow detection of chromosomal aneuploidyandstructuralrearrangementsoccurringinpre-malignantand malignantlesionsduringtumorprogression and invasion. These high resolution molecular cytogenetic techniques are used for genetic screening of single cells in pre-malignant and precursor malignant lesions as well as in exfoliated cells from body fluids and excreta. Consequently, molecular cytogenetic testing offers the promise of an extremely powerful method of risk assessment and early detection of cancer.