A family of elementary particles whose first three members were discovered in 1977. The upsilon mesons, Υ, are the heaviest known vector mesons, with masses greater than 10 times that of the proton. They are bound states of a heavy quark and its antiquark. The quarks which bind to form the upsilons carry a new quantum number called beauty or bottomness, and they are called b-quarks or b. The mass of the b-quark is around 5 GeV. The anti-b-quark or b carries antibeauty, and therefore the upsilons carry no beauty and are often called hidden beauty states. Direct proof of the existence of the b-quark was obtained by observing the existence of B mesons which consist of a b-quark bound to a lighter quark. Twelve bb-mesons have also been observed thus far.