HI 21cm absorption spectroscopy provides an independent probe of the nature of damped Lyman-$\alpha$ systems (DLAs), yielding information on their ISM conditions, kinematics, and spatial extent. We find that the majority of DLAs have significantly higher spin temperatures than seen in the Milky Way and local spirals, indicating larger fractions of the warm phase of HI, especially at high redshifts. Present data suggests that the spin temperature might be used as a secondary indicator of the nature of the absorbing galaxy. We have succeeded in spatially resolving the 21cm absorption in two $z\sim 0.4$ DLAs, thus obtaining lower limits on the transverse extent of these absorbers.