This chapter delves into the quantum circuit model, a primary framework for quantum computation. It begins with the qubit, exploring its representation on the Bloch sphere and its probabilistic measurement outcomes. Quantum gates are introduced as the basic operational units, transforming qubits via unitary operations. The chapter discusses single- and two-qubit gates, building up to universal quantum computation, which enables any quantum function to be constructed through a finite set of gates. This chapter provides an in-depth understanding of information processing in quantum circuits, establishing a practical foundation for executing quantum algorithms and advancing to topics like entanglement-based operations and fault-tolerant design in later chapters.