This course explores atom-photon interactions with a semi-classical treatment: how does a quantum system respond to a classical drive field? We begin by discussing how an atom driven by an optical field reduces to a dipole interaction Hamiltonian. The atom-photon problem can then be mapped onto a spin one-half electron in a magnetic field, since both are driven two-level quantum systems. We develop the Bloch equations, Rabi oscillations, and magnetic resonance. Returning to the optical regime a treatment using density matrices is necessary to include the effects of damping. Dynamics of the density operator are described by the optical Bloch equations, with which one can understand a wide range of current experiments in atomic, molecular, and optical physics and solid-state physics. These quantum dynamics are contrasted to classical (Lorentz-model) dynamics, such as quantum saturation. In the context of a diagonalized atom-photon Hamiltonian, we discuss inversion, dressed states, and light shifts. Applications of this foundational material include electromagnetically induced transparency, slow light, dark states, and laser cooling.