In this chapter, we cover the basic concepts of quantum mechanics that are required to understand the interaction of electromagnetic radiation with matter. The chapter begins with a review of the Planck theory of temperature radiation and the De Broglie[aut]De Broglie, Louis equation relating wavelength to particle momentum. The basis for the time-dependent Schrödinger equationSchrödinger equation is introduced, and its structure is motivated by comparing the dispersion relationDispersion relation for quantum waves with the dispersion relationDispersion relation associated with the classical wave equationWaveequation. The space and time variables in the time-dependent Schrödinger equation are separated, and the resulting time-independent Schrödinger equationSchrödinger equation is solved for the case of a square well potentialSquare well potential.