Hydraulic systems are widely used in mobile machinery. As technology advances, especially with the convergence of the electronics and hydraulic systems, mobile hydraulic systems are becoming more sophisticated. No matter how complicated a particular hydraulic system is, it is often integrated by a few basic circuits. A basic circuit refers to the most simple hydraulic system composed of a few pertinent components and capable of performing a specific operational function. According to their functionality in a hydraulic system, these basic circuits can be classified as pressure-control, directioncontrol, speed-control, sequence-control, and synchronizing-control circuits. The readers should realize that all the circuits introduced here are only used primarily for illustration purposes and may not be directly applicable for practical uses. Engineering design works are often required to make those basic circuits practical and applicable, and there are many more different designs of circuits being used in actual applications.