Technical Abstract: Modern advances in molecular techniques are only recently being incorporated into programs for the classical biological control of weeds. Molecular analyses are able to elucidate information about target weeds that is critical to improving control success, such as taxonomic clarification, evidence of hybridization and cryptic species, better development of test plant lists, and population structure and origins of invasions. Similarly, molecular approaches can improve our knowledge of biological control agents, providing better taxonomic clarity, identification of immature arthropods and fungal pathogens, and levels of genetic variability in agents. Molecular tools also allow easier identification of host associations and provide a tool for post-release evaluation and tracking of agents. This review provides an overview of how to use molecular approaches in biological control of weeds, to assist the adoption of these techniques, and to facilitate fruitful collaboration between scientists studying the biology and ecology of agents and their targets and those with skills using molecular approaches. We describe the current toolbox of molecular techniques relevant to classical biological control of weeds, instruct how to collect field materials for molecular analyses, and give recent examples of the use of molecular methods in biological control of weeds, with comments on the most appropriate methods for analysis of molecular data.