This paper examines the physical characteristics and dynamics of Atlantic tropical instability vortices based on observational data from 1997 to 2000 and a high-resolution numerical model simulation. Most prominent during boreal summer, the vortices are characterized by 400 km spatial scales, westward translation at 0--40 cm s-1, anticyclonic circumferential velocities of 30--110 cm s-1, and SST anomalies that decrease from 2¿C in midbasin to 0.5¿C or less near the western boundary. In contrast, the sea level anomalies grow as the vortices progress westward, reaching maximum amplitudes of 14 cm near 40¿W. The large circumferential velocities, leading to relative vorticities of -4 ¿ 10-6 s-1 and Rossby numbers of 0.5, indicate that centrifugal effects may play an important role in the vortices' vorticity balance. We address the vortices' vertical structure and vorticity dynamics by examination of a high-resolution numerical model. There is a reasonably good agreement between the model and observations. Simulated vortices are confined mostly to the mixed layer, which increases westward in depth from 30 to 100 m near the western boundary. In the eastern basin, potential vorticity within the vortices is not conserved but decreases because of wind stress forcing. In contrast, in the western basin, decreases in potential vorticity are due mostly to the vortices' acquisition of Southern Hemisphere water. We estimate an annualized cross-equatorial transport of up to 1.2 Sv associated with the vortices, which is eventually contributed to the North Brazil Current system.