It is now commonly accepted shock acceleration (in various forms) is an important (if not the most) mechanism to accelerate energetic particles or cosmic rays. One of the attractive features of this mechanism is its efficiency. However, simple energy budget argument then leads to the consideration of the backreaction of the cosmic rays on the plasma and the shock. Hydrodynamic approach is a good approximation in studying the structure of the modified shock. There are two generic structures. One of them is the upstream plasma is slowed down and is followed by a sub-shock, and the other is a smooth transition. Various aspects of the modified shock, such as instability, injection, efficiency, time evolution, are discussed. Remarks on the limitation of hydrodynamic approach are given.
