Helium nanodroplets: An overview

We review the physics of helium nanodroplets with emphasis on theoretical aspects that have been the subject of recent research, like He-3 and mixed He-3- He-4 droplets, vortices in droplets and droplets on weakly attractive substrates, and in others that have not been previously reviewed in detail. Whenever possible, we compare results from different theoretical methods, with the aim of assessing the capabilities and limitations of the more phenomenological ones, that often constitute the only instrument to address some of the questions raised by current experiments.