Hydraulic systems are widely used on industry and mobile machinery, in the form of various circuits, to achieve desired functionalities. No matter how complicated a particular hydraulic system is, it is often integrated by a few basic circuits. A basic circuit refers to the simplest hydraulic system, which is composed of a few relevant components and is capable of performing a specific operational function. According to their functionality in a hydraulic system, these basic circuits can be classified as pressure control, direction control, speed control, sequence control, and synchronizing control circuits. This chapter explains how those basic circuits are constructed and work. This chapter also introduces a few special-function circuits that often find their applications in more sophisticated applications, as well as a few integrated circuits to show how those basic circuits are integrated in a comprehensive system to perform the designated work.