Chapter 2

Pulsars are unique and versatile objects that can be used to study an extremely wide range of physical and astrophysical problems. In many respects, pulsars are a physicist's dream come true: besides testing theories of gravity one can study the gravitational potential and magnetic field of the Galaxy, the interstellar medium, stars, binary systems and their evolution, solid state physics and the interiors of neutron stars. Investigating the radio emission of pulsars provides insight into plasma physics under extreme conditions. This subject is particularly difficult, since a proper theoretical description of the observations is still lacking. Fortunately, for most applications, it is not necessary to understand how pulsars work; instead, it is sufficient usually to consider them as natural clocks emitting highly polarised coherent signals. To motivate the techniques described in subsequent chapters, we outline now a selection of the most wide-ranging applications of pulsars so far.