The first in situ measurements of the details of the global composition and dynamics of the ionosphere of Venus have been obtained from the Bennett ion mass spectrometer on the Pioneer Venus orbiter during the period December 1978 through August 1979. These results include observations of three related plasma regimes, including (1) the bowshock-ionosheath region, (2) the thermal ionosphere, and (3) a superthermal flowing ion layer interfacing with the ionosphere at the ionopause and extending outward to variable heights above the planet. During quiet periods an abundant ionosphere is observed both on the dayside and on the nightside, generally dominated by O+ above 200 km (except for a predawn region where H+ exceeds O+) and by O2+ down to typical periapsis heights of about 160 km. A sampling of some of the less disturbed data exhibits strong day to night variations in the distributions of the most prominent ions including O+, O2+, CO2+, C+, N+, CO+(N2+), NO+, H+, He+, O18+, O++, and H2+. Important features of the day-night variation measured at 200 km and a fixed latitude of about 8¿N include asymmetric nightside bulges in the distributions of the light ions H+ and He+, peaking near dawn at 110¿ and 90¿ solar zenith angle, respectively. These asymmetries are associated with dawn-dusk asymmetries in the d distributions of O+, O2+, and other molecular ions, which exist in higher concentrations near dusk, relative to dawn. Associated observations of the bowshock-ionosheath and the ionosphere-superthermal plasma layer regions indicate close coupling among these plasma regimes. The ionopause, identified as the boundary between the thermal ionosphere and the superthermal flow layer, is encountered near 250--400 km near the subsolar point and extends at times to heights greater than 100 km in the flanks and nightside regions. Under disturbed nightside conditions, particularly noticeable in the dusk region, the ionosphere may exhibit randomly spaced concentration gradients of an order of magnitude associated with complex patterns of ion flow with velocities up to 10 km/s encounted within the main body of the ionosphere.