Ab initio molecular-dynamics (MD) simulations are increasingly being used to study defects in silicon. Simulated quenching and/or conjugate gradient calculations are now common tools to explore the minima of complicated potential energy surfaces. However, the real dynamics are in the constant-temperature runs. These highly computer-intensive simulations are still limited to real times of the order of picoseconds. However, they provide a fantastic window into processes that are beyond the reach of static (T=0K) calculations. In this paper, the challenges of constant-temperature ab initio MD simulations are discussed and examples given: the formation of H2*, the diffusion of small self-interstitial clusters, the restless interstitial H2 molecule, and the dynamic calculation of vibrational frequencies from the velocity-velocity autocorrelation function. The results are obtained using methods based on Sankey's 'ab initio tight-binding' approach, with atomic-like basis sets rather than plane waves.