Manifestations of Quantum Physics at the thermodynamical level are found in a broad range of physical systems. A famous example is superfluidity, discovered at the beginning of the 20th century and found in many different situations, from liquid helium to neutron stars. Dilute ultracold gases offer a unique versatility to engineer quantum many-body systems, which can be directly compared with theory thanks to the high controllability of their environment. In this thesis we present experimental investigations led on ultracold lithium gases. Lithium provides the possibility to study ensembles of bosons and fermions, with controllable interactions between the constituents. We present experimental techniques for preparation and studies of degenerate gases of lithium. Applying these techniques led us to the first experimental realization of a mixture of a Bose superfluid with a Fermi superfluid. We demonstrate that both components are in the superfluid regime, and that the counter-flow motion between them possesses the characteristics of superfluid flow, with the absence of viscosity below a critical velocity, and an onset of friction above. Using collective oscillations of the mixture, we measured the coupling between the two superfluids in close agreement with a theoretical model.