In this review, gas-phase chemistry of interstellar media and some planetary atmospheres is extended to include molecular complexes. Although the composition, density, and temperature of the environments discussed are very different, molecular complexes have recently been considered as potential contributors to chemistry. The complexes reviewed include strongly bound aggregates of molecules with ions, intermediate-strength hydrogen bonded complexes (primarily hydrates), and weakly bonded van der Waals molecules. In low-density, low-temperature environments characteristic of giant molecular clouds, molecular synthesis, known to involve gas-phase ion-molecule reactions and chemistry at the surface of dust and ice grains is extended here to involve molecular ionic clusters. At the high density and high temperatures found on planetary atmospheres, molecular complexes contribute to both atmospheric chemistry and climate. Using the observational, laboratory, and theoretical database, the role of molecular complexes in close and far away is discussed.P lanetary atmospheres and interstellar media have traditionally been lumped together; they are complex dynamical entities whose understanding has been and continues to be a challenge. That this subject is new may be appreciated by the fact that the knowledge that hydrogen is the abundant element of the universe is less than 100 years old. Hydrogen and helium comprise more than 99% of the universe. Thus, interstellar chemistry must be quite different from that of the atmospheres of the minor planets Mars, Earth, and Venus. Conceding the dominance of hydrogen and helium on the cosmic scale and even in the major planets, more complex molecules than H 2 attract our attention and dominate our thinking (1-4).An understanding of the chemistry of the universe must involve a wide range of processes. A bit of history with respect to interstellar chemistry is relevant to our perspective. The observed interstellar molecules, CH, CH ϩ , and CN, are quantitatively discussed by Herbig (5) in his classic study of z Oph. These species have optical transitions in spectral regions where Earth's atmosphere is transparent. It was assumed that in interstellar clouds, which were translucent and thus bathed in the galactic radiation field, the major components were atomic. Because the average density of the galaxy is 1 atom cm Ϫ1 (6) three-body processes were readily dismissed. Kramers and ter Haar (7) in 1946 presented a calculation for the formation of CH by the radiative association of C and H, i.e., C ϩ H ϭ CH ϩ photon. In 1951 Bates and Spitzer (8) showed that the CH calculated formation rate was seriously incorrect and with that ruled out synthesis by gas-phase association. Their concluding statement ushered in a lengthy period with emphasis on molecular synthesis on grain surfaces under a variety of conditions (3,(9)(10)(11).With the advent of molecular radioastronomy (12) the molecular character of giant molecular clouds could be examined. These are the largest structures within the gala...