This textbook offers clear explanations of the theory of optical spectroscopic phenomena and shows how these ideas are used in modern molecular and cellular biophysics and biochemistry. The topics covered include electronic and vibrational absorption, fluorescence, resonance energy transfer, exciton interactions, circular dichroism, coherence and dephasing, ultrafast pump-probe and photon-echo spectroscopy, single-molecule and fluorescence-correlation spectroscopy, Raman scattering, and multiphoton absorption. The explanations are based on time-dependent quantum mechanics and are sufficiently thorough and detailed to be useful for both students and researchers. With the clear, thoroughly illustrated explanations that begin from first principles, the text will also be accessible to readers with little prior training in quantum mechanics. Extra details and highlights are featured in special boxes throughout the text. A helpful exercises section has been included to this new student edition.