This chapter discusses excitonic and plasma effects of InAs and In1-yGayAs quantum wells. Information about band offsets and related topics are presented. Data of optical properties are presented for low- and medium-density regimes. Comparison of transmission, reflection and photoluminescence (PL) spectra of InAs and In1-yGayAs/InP multiple quantum well (MQW) sample are illustrated. Luminescence of a MQW embedded in a cavity with dielectric mirrors is shown. Localization is an important issue in In1-yGayAs QW since there is alloy disorder in both barrier and well apart from well width fluctuations, depending on the composition. Optical nonlinearities and dynamics of excitons, biexcitons and trions in intermediate-density are provided. Influence of external fields on excitons (and related quasi-particles) is reviewed in the low- and medium-density regimes. The data for unipolar electron or hole plasmas which are generally produced by strong (modulation) doping and for bipolar electron-hole plasmas created by strong optical pumping or by carrier injection in a forward biased p(-i-)n junction are presented. Band gap renormalization of the first three heavy-hole interband transitions is plotted as a function of carrier density for a 15nm wide In1-yGayAs/InP SQW. Finally, this section illustrates changes in transmission of a 10nm InGaAs SQW in a modulation doped field effect transistor (MODFET) structure. Dynamical properties of the plasma effects proceeding from fast to slow are reviewed.