Side-channel cryptanalysis is a new research area in applied cryptography that has gained more and more interest since the mid-nineties. It considers adversaries trying to take advantage of the physical specificities of actual cryptographic devices. These implementation-specific attacks frequently turn out to be much more efficient than the best known cryptanalytic attacks against the underlying primitive seen as an idealized object. This chapter aims to introduce such attacks with illustrative examples and to put forward a number of practical concerns related to their implementation and countermeasures.