I will give a brief introduction to the photophysics of single-walled carbon nanotubes and how their quasi one-dimensional structure enables the observation of quasiparticles such as excitons, polarons and trions at room temperature by simple optical absorption and photoluminescence spectroscopy in the near-infrared. A special focus will be the dependence of these optical properties and of the formation and decay of excitons and trions on charge carrier density, which can be tuned widely by chemical, electrochemical and electrostatic doping.