Collisionless Shocks (CSs) are found in almost every plasma environment observed either directly or indirectly from labs to supecrlusters. They are the most effective accelerators of charged particles.CSs are strongly nonlinear objects where the field-particle interactions are crucial. Understanding CSs is necessary for knowledge of their outstanding role in the activity of most of the known objects in the universe and is relevant to the most fundamental phenomena in space plasmas such as acceleration, multi-scale processes, dissipation, and transition to irreversibility.