  An overview is presented of experiments on ballistic electrical transport in
inhomogeneous superconducting systems which are controlled by the process of
Andreev reflection. The initial experiments based on the coexistence of a
normal phase and a superconducting phase in the intermediate state led to the
concept itself. It was followed by a focus on geometrically inhomogeneous
systems like point contacts, which provided a very clear manifestation of the
energy- and directional dependence of the Andreev reflection process. The point
contacts have in recent years evolved towards the atomic scale by using
mechanical break-junctions, revealing in a very detailed way the dependence of
Andreev reflection on the macroscopic phase of the superconducting state. In
present day research the superconducting inhomogeneity is constructed by clean
room technology and combines superconducting materials with, for example,
low-dimensional materials and topological insulators. Alternatively the
superconductor is combined with nano-objects, such as graphene, carbon
nanotubes, or semiconducting nanowires. Each of these 'inhomogeneous systems'
provides a very interesting range of properties, all rooted in some
manifestation of Andreev reflection.
