Magellan radar images and altimetry data and Pioneer Venus gravity data of major highlands and lowlands on Venus are examined with the objective of relating these tectonic and volcanic landforms to convection within Venus' mantle. Two roughly circular lowlands, Atalanta and Lavinia planitiae, are bowl-shaped depressions which contain compressional tectonic features and lack hotspot-related features such as coronae and shield volcanoes. They are identified as surface expressions of young mantle coldspots or regions of mantle downwelling. Highlands can be divided into two distinct groups: volcanic rises and plateau-shaped highlands. Volcanic rises are domical, circular to elongate regions characterized by volcanic construction; extensional tectonism; large, positive gravity and geoid anomalies centered on the highlands; and large apparent depths of compensation. Volcanic rises include Beta, Bell, Atla, and Western Eistla regiones and are identified as surface expressions of large mantle hotspots or plumes. Plateau-shaped highlands are steep-sided elevated regions that range from circular (Alpha Regio) to elongate or irregular (Ovda Regio, Tellus Regio) in shape. Their surfaces are dominated by complex ridged terrain, rather than the shield volcanoes and flows that dominate volcanic rises. Most plateau-shaped highlands contain compressional structures, some of which commonly lie along their margins, and have small gravity and geoid anomalies and small apparent depths of compensation compared to volcanic rises. Plateau-shaped highlands, which include those mentioned above and Western Ishtar Terra, Thetis Regio, and Phoebe Regio, are identified as surface expressions of mantle coldspots or regions of downwelling. The highland coldspot features are elevated as a consequence of crustal thickening; crustal thickening is absent or minor at lowland coldspots. The importance of mantle downwelling in the tectonic deformation of Venus suggests that its mantle, like the mantle of Earth, is strongly heated from within. The major Venusian hotspots attest to the existence of large mantle plumes carrying heat upward from the planet's core.