We present a review of the use of molecular dynamics techniques to study the behaviour of proteins. The application of such methods to biological macromolecules has evolved directly from its use to study simpler physical and chemical systems. We describe the methods typically used in producing multiple nanosecond atomic trajectories. This technique is now so common that it is impossible to review the whole area. Therefore, we have focused on three areas, namely the application to proteins of biomedical importance, to folding of proteins from a random conformation to a stable well-defined tertiary structure and to the reverse process, that of unfolding. Finally, we describe some methods which have been developed to analyse complex trajectories with the aim of defining the most important features of protein dynamics and changes in conformation.