group crossing times (Barnes 1989; Carnevali, Cavaliere & Santangelo 1981; Cavaliere et al.  1983; Governato, Bhatia & Chincarini 1991). To resolve this problem, Mamon (1986) suggested that a large fraction of compact groups are merely unbound chance superpositions of galaxies within loose groups (Mamon 1986, 1987). Hernquist, Katz & Weinberg (1995) suggested that compact groups are unbound superpositions of galaxies viewed along filaments. In contrast, simulations by Hickson & Rood (1988) and Diaferio, Geller & Ramella (1995) and observational work by Zepf (1993) and Pildis, Bregman & Schombert (1995) suggest that many of Hickson’s compact groups (HCG’s, hereafter) are, in fact, bound systems.

The short crossing times of HCG’s are not a problem if the groups form continually in dense environments like loose groups (Barnes 1989, Diaferio, Geller & Ramella 1994). Studies of the environments around some of the HCG’s reveal that many of them are indeed embedded in larger, looser systems (Ramella et al.  1994; de Carvalho, Ribeiro & Zepf 1994).

Compact groups have implications for cosmology, the development of large-scale structure and the evolution of the galaxy population. Compact groups are more likely to form during the present epoch in a dense universe (Diaferio 1994, Governato, Tozzi & 