Parthenium (Parthenium hysterophorus L.), a member of Asteraceae, is an obnoxious national weed of wide ecological niches including waste land and agroecosystems. The weed has phenolics, flavonoids, alkaloids, pseudoguaianolides and oils, many of which have been implicated in allelopathy. The phenolics include caffeic acid, vanillic acid, ferulic acid, chlorogenic acid, p-coumaric acid and p-hydroxy benzoic acid. The plant and its parts owe various properties involving allelopathy, phytotoxicity, herbicidal activity, growth regulation / inhibition, including on nitrification and nitrifying bacteria. Among flavonoids are quercetagetin-3, 7-dimethyl ether, 6-hydroxy kempferol-3, 7-dimethyl ether, kaempferol 3-O-glucoside, quercetin 3-O-glucoside, kaempferol-3-O-glucoarabinoside, and lignan (+) syringaresinol. They confer antioxidant activities, scavenging effects on activated carcinogens and mutagens, action on cell cycle progression, altered gene expression, UV-B protection in plants, warding off microbial infections, and protection of plants from herbivores, etc. The alkaloids have been detected but yet to be identified. The pseudoguaianolides identified from the species include parthenin, anhydroparthenin, ambrosin, coronopillin, damsin, hymanin, 8-b-hydroxyparthenin, 2b-hydroxycoronopilin, tetraneurin-A, ambrosanolides, charminarone, 8-b-acetoxyhysterone C, deacetyltetraneurin A, hysterin, hysterone E, hysterone D, conchasin A, acetylated pseudoguaianolides, scopoletin (belongs to coumarin), and dihydroxyparthenin. They have diverse activities like cytotoxic, antitumor, antibacterial, antifungal, phytotoxic, antiprotozoan, active against human and animal parasites (including intermediate hosts), insecticidal, moluscicidal, vertebrate feeding deterrence and toxicity, allergic contact dermatitis, mitochondrial oxidative phosphorylation inhibition, allelopathic, anti-inflammatory, and antimalarial. The oils identified from the species include a-pinene, camphene, b-pinene, sabinene, b-myrcene, b-terpene, limolene, b-ocimene, ocimene, p-cymene, linalool, caryophyllene, humulene, terpinene-4-ol and many unidentified compounds. They have been shown to be antifungal, antibacterial, antimicrobial, virucidal, antiparasitical, insecticidal, medicinal, cytotoxic and many find use in cosmetics. Information on role of specific constituents in allelopathic interaction of the species in natural ecosystems still appears to be scarce. Bioherbicidal potential of most of the constituents has not been investigated. These are attractive areas with potential of facilitating development of newer herbicides or pesticides.