  The combination of different kinds of ultracold gases constitutes a novel
powerful experimental framework for the investigation of a variety of physical
problems. We illustrate the differences among possible quantum mixtures, be
they homonuclear spin mixtures or heteronuclear ones, and show how they can be
exploited to investigate a plethora of topics from the few-body to the
many-body regimes. In particular, we discuss quantum mixtures of ultracold
gases under three different perspectives: systems made of a few atoms of
different kinds, single impurities immersed in a host quantum gas, and quantum
mixtures of two interacting gases. Given the broad spectrum of possible topics
and experimental regimes, in this review we restrict the discussion on single
harmonic or flat traps, predominantly in a three-dimensional configuration. A
selection of results on recent experiments and possible interesting future
directions are given.
