Introduction

Open clusters provide the astronomer with a rich source of objects for studying stellar structure over the full mass range of stable, hydrogen burning stars; furthermore, stellar evolution can be studied as the higher mass stars evolve away from, and as the low mass stars contract onto, the main sequence. Moreover, open cluster studies of objects that have too low a mass to stabilise on the hydrogen burning main sequence (i.e. brown dwarfs) have recently come of age, so now it is possible to study the physics of coeval objects having masses ranging over three orders of magnitude (and luminosities over eight orders of magnitude). Properties of very low mass (VLM) stars being studied in open clusters include lithium evolution, angular momentum evolution, spotting and variability, choronal activity, the binary fraction, and, most fundamentally, the mass function.