  Energetic ions have been observed since the very first laser-plasma
experiments.Their origin was found to be the charge separation of electrons
heated by thelaser, which transfers energy to the ions accelerated in the
field. The adventof ultra-intense lasers with pulse lengths in the femtosecond
regime resulted inthe discovery of very energetic ions with characteristics
quite different fromthose driven by long-pulse lasers. Discovered in the late
1990s, these ion beamshave become the focus of intense research worldwide,
because of their uniqueproperties and high particle numbers. Based on their
non-isotropic, beam-likebehaviour, which is always perpendicular to the
emitting surface, theacceleration mechanism is called target normal sheath
acceleration (TNSA). Weaddress the physics of the mechanism and its dependence
on laser and targetparameters. Techniques to explore and diagnose the beams, to
make them usefulfor applications, are also addressed.
