Current plate motions can be accounted for by a balance of active forces, slab pull, ridge push, and, for continental plates, trench suction, with drag beneath the plate as a resistive force. If we assume that the same forces have acted through time, we can reconstruct plate motions from the geometry of past plate boundaries. Paleozoic reconstructions are made with paleomagnetic, tectonic, climatic, and biogeographic data, as no ocean floor remains. PALEOMAP reconstructions are used to estimate past plate speeds and to test simple dynamical models in order to determine which ranges of forces best accounts for the observations. Over the last 600 m.y., plate speeds averaged over 40- to 100-m.y. intervals show considerable variation; Gondwana's speed oscillates from 20 to 60 km/m.y. over a long timescale (200--400 m.y.) with considerable noise superposed. Over the Paleozoic Era motions for large continental regions average 28 km/m.y.; force balance models based on present-day observations suggest that continental regions without a large attached slab would move 30 mm/yr. The opening and closing of the ocean between Laurentia and Gondwana 560--400 Ma is used to test dynamical models and the parameter values assumed. In the late Precambrian, Laurentia rifted away from Gondwana. In the earliest Cambrian it was near 40¿S; by Late Cambrian and Ordovician it had moved to the equator. During the Silurian and Devonian, Laurentia reversed direction and later collided with Gondwana at 40¿S. In a model of the forces acting on the plates, slab pull, ridge push, and trench suction are assumed to balance plate drag. Only certain ranges of ridge-push and trench parameters can model both the opening and subsequent closing of the ocean. The dynamic models, with parameter values inferred from present rates, bracket the rates required by the reconstructions. ¿ American Geophysical Union 1995