Lyman alpha as an Astrophysical and Cosmological Tool : Saas Fee Advanced Course 46. Swiss Society for Astrophysics and Astronomy The hydrogen Lyman alpha line is of utmost importance to many fields of astrophysics. This UV line being conveniently redshifted with distance to the visible and even near infrared wavelength ranges it is observable from the ground and provides the main observational window on the formation and evolution of high redshift galaxies. Absorbing systems that would otherwise go unnoticed are revealed through the Lyman alpha forest Lyman limit and damped Lyman alpha systems tracing the distribution of baryonic matter on large scales and its chemical enrichment. We are living an exciting epoch with the advent of new instruments and facilities on board of satellites and on the ground. Wide field and very sensitive integral field spectrographs are becoming available on the ground such as MUSE at the ESO VLT. The giant E ELT and TMT telescopes will foster a quantum leap in sensitivity and both spatial and spectroscopic resolution to the point of being able perhaps to measure directly the acceleration of the Hubble flow. In space the JWST will open new possibilities to study the Lyman alpha emission of primordial galaxies in the near infrared. As long as the Hubble Space Telescope will remain available the UV restframe properties of nearby galaxies will be accessible to our knowledge. Therefore this Saas Fee course appears very timely and should meet the interest of many young researchers Physics of Lya Radiative Transfer HI Absorption in the Intergalactic Medium Observations of Lya Emitters at High Redshift Lyman alpha Emission and Absorption in Local Galaxies Index