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Neptune is the outermost of the four giant planets in our solar system. The region in which Neptune orbits has longer dynamical time scales and probably had a lower surface mass density of solids than zones closer to the Sun. Both of these properties suggest that Neptune took significantly longer to accrete than did the other giant planets. "Runaway accretion" may permit growth of isolated ~ 10 M_⊕ cores in the outer solar system on time scales of 10^7 to 10^8 yr. It remains unclear, however, whether Neptune's core could have grown massive enough to trap > 1 M_⊕ of H_2 and He before the protoplanetary disk was dispersed. If core formation did occur on that time scale, models for the accretion of gas onto a growing condensible element core provide insights into nebular conditions required for the growth of Neptune-like planets. An alternative model is that ice-rich planetesimals of mass ≳ 0.1 M_⊕ trapped substantial quantities of solar nebula gas mixed with evaporated steam in high mean molecular weight atmospheres. The final stages of Neptune's growth could then have occurred after dispersal of the solar nebula, alleviating the possible time scale problem associated with the single core model. The observed similarity of the condensible element masses of four giant planets in our solar system is not an obvious consequence of current models. The decrease in H_2 and He masses from Jupiter to Saturn to Uranus and Neptune may be understood in terms of the dispersal of the solar nebula. However, several other dynamical problems remain unsolved. In particular, dynamical models of planetary growth which have been successfully applied to the terrestrial region produce far less satisfying results in the Uranus-Neptune region. The larger planetary masses and weaker influence of solar gravity cause mutual scattering to spread planetary embryos over a wide range of heliocentric distance. Several residual planets much more massive than Pluto become isolated in trans-Neptunian orb

Although Earth-based observations provide hints of Neptune's dynamic activity, most of the observations of cloud patterns, winds, and horizontal variations in temperature are from Voyager. The wind speed varies from 400 m s^(-1) westward at the equator to 250m s^(-1) eastward at -70° latitude. As on all the giant planets, the winds decay with height in the stratosphere. The meridional contrasts in temperature are small in the upper troposphere and lower stratosphere, with mid-latitude minima that are reminiscent of those on Uranus. The oscillations and motions of Neptune's large spots are more regular than those of Jupiter and Saturn. During the Voyager observations, the Great Dark Spot moved steadily equatorward while it oscillated in shape with an 8-day period. The latitude and longitude of Dark Spot 2 oscillated with a 36-day period. The small elements within each major feature appeared and disappeared in less than a day. Such activity is remarkable for a planet whose emitted power per unit area is 1/20 that of Jupiter and 1/400 that of the Earth. Weak viscosity can account for the decay of winds in the stratosphere, and models without viscosity can account for the spot oscillations. But the wind velocities and temperatures vs latitude--especially the differences among the giant planets--have not been explained. Major unknowns concern convection and latent heat release, the interaction with Neptune's fluid interior, and the importance of internal energy relative to solar energy in driving the circulation.

Neptune, together with Uranus, belongs to the ice-rich class of Jovian planets. Although molecular hydrogen is the dominant species in its atmosphere, all available evidence points toward an interior composition dominated by the planetary ices, primarily water. In some models, small amounts of metallic hydrogen may also be present in deeper layers. If a rock component (magnesium-silicates and iron) is present, it is either underabundant with respect to solar composition, or is not strongly differentiated from the ice component. Neptune interior models which fit constraints imposed by atmospheric chemistry, mean density, and gravitational harmonics, strongly resemble Uranus models. Yet Neptune and Uranus differ substantially in their interior heat flow budgets. While Neptune's intrinsic heat flow is reliably detected, only an upper limit is available for the intrinsic heat flow of Uranus. Possible important influences on interior heat flow include solar heating at the upper boundary, and compositional gradients in the interior. The potential for hydromagnetic dynamo action in the interior depends on the vigor of convection in electrically conducting regions. Electrical conductivity in the icy material is adequate for dynamo action if temperatures within Neptune are comparable to those achieved in the shock experiments. Although the zonal gravitational harmonics J_2 and J_4 provide important clues to interior structure, for a given pressure-density relation the J_4 harmonic can be substantially reduced in absolute value by the observed atmospheric equatorial subrotation, if the subrotation involves significant mass.