The healthiness of a vegetable cannot solely be inferred from the amount of health-promoting compounds in the raw materials. Brassica vegetables, for example, are consumed mostly after processing to improve palatability and to extend the shelf life. However, processing also results to various changes in the content of glucosinolates which intakes are associated with a reduced risk of several cancers. The large variety in cooking practices and processing methods affect the glucosinolate content in the vegetables, particularly due to processes that allow for enzymatic hydrolysis and thermal degradation of glucosinolates, and leaching of the bioactive components. Knowledge on the effect of preparation and processing of Brassica vegetables is important to evaluate the healthiness of the consumed product and to investigate mechanisms to retain high glucosinolate levels at the stage of consumption and to increase the intake of health-protective compounds by the consumer. By using a mechanistic approach, the fate of glucosinolates during different processing and preparation methods and conditions can be explained. Boiling and blanching reduce the glucosinolate content significantly particularly because of the mechanisms of leaching following cell lysis and diffusion, and partly due to thermal and enzymatic degradation. Steaming, microwave processing, and stir frying either retain or only slightly reduce the glucosinolate content due to low degrees of leaching. These methods can enhance the accessibility of glucosinolates from the plant tissue. Fermentation reduces the glucosinolate content considerably, the underlying mechanisms are not yet completely clear, but enzymatic breakdown seems to play an important role. Studying the changes of glucosinolates during processing by a mechanistic approach is shown to be valuable to redesign the processing and to reformulate the product for improving health benefits of these compounds.