The development of fluorescence in situ hybridization (FISH) has led to the advancement of chromosome studies not only for physical mapping and genome analyses but also as a tool for evolutionary studies. Isolated repetitive DNA sequences have been useful cytological markers, but large-insert genomic libraries (e.g., bacterial artificial chromosomes [BACs], yeast artificial chromosomes [YACs], and cosmids) are being increasingly used to serve as probes for large segments of DNA across related genomes. Although FISH is usually applied to metaphase chromosomes, fiber FISH, a variation of FISH using extended DNA fibers, is now used to measure loci at the resolution of a few kilobases to compare orthologous genome segments across related genomes. A generalized set of protocols for chromosome preparation, FISH, and fiber FISH are presented; however, it is often necessary to experiment with techniques for different plant taxa for successful molecular cytogenetic studies.