  We analyse some properties of circumplanetary discs. Flow through such discs
may provide most of the mass to gas giant planets, and such discs are likely
sites for the formation of regular satellites. We model these discs as
accretion discs subject to the tidal forces of the central star. The tidal
torques from the star remove the disc angular momentum near the disc outer edge
and permit the accreting disc gas to lose angular momentum at the rate
appropriate for steady accretion. Circumplanetary discs are truncated near the
radius where periodic ballistic orbits cross, where tidal forces on the disc
are strong. This radius occurs at approximately 0.4 r_H for the planet Hill
radius r_H. During the T Tauri stage of disc accretion, the disc is fairly
thick with aspect ratio H/r > 0.2 and the disc edge tapering occurs over a
radial scale ~ H ~ 0.1 r_H. For a circular or slightly eccentric orbit planet,
no significant resonances lie within the main body of the disc. Tidally driven
waves involving resonances nonetheless play an important role in truncating the
disc, especially when it is fairly thick. We model the disc structure using one
dimensional time-dependent and steady-state models and also two dimensional SPH
simulations. The circumplanetary disc structure depends on the variation of the
disc turbulent viscosity with radius and is insensitive to the angular
distribution of the accreting gas. Dead zones may occur within the
circumplanetary disc and result in density structures. If the disc is turbulent
throughout, the predicted disc structure near the location of the regular
Jovian and Saturnian satellites is smooth with no obvious feature that would
favor formation at their current locations.
