A major frontier in medical genetics is the definition of the molecular basis of multifactorial diseases. This is especially relevant in the field of clinical psychiatry where the majority of common disorders display complex inheritance patterns, and are further influenced by environmental interactions. New technologies are needed to help address the pressing needs for discovering and deciphering the nature of such disease-associated genes. One such technology which has emerged within the past 3 years involves hybridization-based nucleic acid array (DNA chip) analysis. This technology has the potential to have a lasting impact on diverse genomic-based applications such as large-scale gene mapping studies, mutational analysis, and global expression level monitoring of all human genes. In this review we will describe the fundamental principles behind nucleic acid array-based assays, while focusing on their applications towards genome-wide DNA and RNA analysis. The current capabilities and limitations of these technologies will be discussed, with a focus on areas where future development will be needed for DNA chip-based assays to achieve their full potential.