  The problem of collisionless shocks is posed as the problem of understanding
how in a completely collisionless streaming high-temperature plasma shocks can
develop at all, forming discontinuous transition layers of thickness much less
than any collisional mean free path length. The history of shock research is
briefly reviewed. It is expressed that collisionless shocks as a realistic
possibility of a state of matter have been realized not earlier than roughly
half a centruy ago. The basic properties of collisionless shocks are noted in
preparing the theory of collisionless shocks and a classification of shocks is
given in terms of their physical properties, which is developed in the
following chapters. The structure of this chapter is as follows: 1. A cursory
historical overview, describing the early history, gasdynamic shocks, the
realisation of the existence of collisionless shocks and their investigation
over three decades in theory and observation until the numerical simulation
age, 2. Posing the shock problem as the question: When are shocks? 3. Types of
collisionless shocks, describing electrostatic shocks, magnetized shocks, MHD
shocks, shock evolutionarity and coplanarity, switch-on and switch-off shocks,
4. Criticality, describing the transition from subcritical dissipative to
supercritical viscose shocks, 5. Remarks.
