This chapter discusses quantum-well structures of GaAs, Al1-yGayAs . The chapter presents spectra, effects and data for the low- and intermediate-density regimes in which excitons (X), biexcitons (or excitonic molecules, XX) and trions (two electrons and one hole or vice versa X-, X+) are still good quasiparticles. First, it presents some selected spectra of excitons in GaAs quantum wells including absorption spectra with the related techniques of photoluminescence - and photocurrent excitation – spectroscopy (PLE and PCE, respectively) and also reflection and luminescence spectra. The chapter describes the following concepts: linear optical spectra including excitons, defects, disorder and interexcitonic transitions, exciton binding energy, exciton polaritons in quantum wells and waveguides (including the exciton fine structure), cavity polaritons, localized and bound excitons, and exciton transport. The chapter finally discusses Raman scattering, and optical nonlinearities and dynamic properties of excitons, biexcitons and trions.