Brassicaceae () or (the older) Cruciferae () is a medium-sized and economically important family of flowering plants commonly known as the mustards, the crucifers, or the cabbage family. Most are herbaceous plants, while some are shrubs. The leaves are simple (although are sometimes deeply incised), lack stipules, and appear alternately on stems or in rosettes. The inflorescences are terminal and lack bracts. The flowers have four free sepals, four free alternating petals, two shorter free stamens and four longer free stamens. The fruit has seeds in rows, divided by a thin wall (or septum).

The family contains 372 genera and 4,060 accepted species. The largest genera are Draba (440 species), Erysimum (261 species), Lepidium (234 species), Cardamine (233 species), and Alyssum (207 species).

The family contains the cruciferous vegetables, including species such as Brassica oleracea (cultivated as cabbage, kale, cauliflower, broccoli and collards), Brassica rapa (turnip, Chinese cabbage, etc.), Brassica napus (rapeseed, etc.), Raphanus sativus (common radish), Armoracia rusticana (horseradish), but also a cut-flower Matthiola (stock) and the model organism Arabidopsis thaliana (thale cress).

Pieris rapae and other butterflies of the family Pieridae are some of the best-known pests of Brassicaceae species planted as commercial crops. Trichoplusia ni (cabbage looper) moth is also becoming increasingly problematic for crucifers due to its resistance to commonly used pest control methods. Some rarer Pieris butterflies, such as P. virginiensis, depend upon native mustards for their survival in their native habitats. Some non-native mustards such as Alliaria petiolata (garlic mustard), an extremely invasive species in the United States, can be toxic to their larvae.