Abstract Ecdysteroids, the molting hormones of arthropods, have been found in many plant species. Phytoecdysteroids belong to a large family of ecdysteroids that comprises more than 250 representatives. To date, ecdysteroids or ecdysteroid-like compounds have been found in gymnosperms, angiosperms, fungi, algae, and certain marine organisms. Ecdysteroids accumulate in various plant organs, including fruits, seeds, flowers, anthers, leaves, and roots, during various phases of their growth. Ecdysteroid concentrations in plants may be very large, i.e., several orders of magnitude higher than in most arthropods. The chemical structure of phytoecdysteroids is very varied. Plants may contain C 27 , C 28 , or C 29 type of ecdysteroids either in the free form or as various polar or apolar conjugates. They may also contain C 24 , C 21 , or C 19 compounds resulting from various side chain cleavage reactions. The presence of ecdysteroids and their conjugates in plants is suggested to be a factor which may alter development of nonphytoecdysteroid-adapted insect species. Phytoecdysteroids induce abnormal molting in many arthropods with lethal effect. Hence these compounds may potentially prove useful to control insect herbivory on agriculturally important plants. However, many functions of ecdysteroids in plants are still unknown. At present, our knowledge of the effects of phytoecdysteroids in animals or human is still rather fragmentary. Phytoecdysteroids from Ajuga decumbens showed strong inhibitory effect on early induction and potent antitumor-promoting activities of Epstein–Barr virus on a mouse skin. It suggests that A . decumbens may be valuable as a source of potential cancer chemopreventive agent. Phytoecdysteroids have an antifungal and antibacterial activity. They are apparently nontoxic to mammal ecdysteroids. A very low toxicity (LD 50 > 6 g/kg) of ecdysteroids in vertebrates is observed. The wide range of beneficial pharmacological (adaptogenic, anabolic, antidiabetic, hepatoprotective, immunoprotective, wound-healing, anti-inflammatory, and perhaps even antitumor) activities is claimed for ecdysteroids. In particular, this has led to a large (and unregulated) market for ecdysteroid-containing preparations for bodybuilders, sportsmen, and pets, among others. Ecdysteroids are also being considered as nutraceutical additives to food products. They have antioxidative and antifree-radical properties.