Contains the worked out lectures given at the 36th Saas-Fee Advanced Course "First Light in the Universe" by three scientists in the field: Abraham Loeb, Andrea Ferrara, and Richard Ellis. This book provides a comprehensive introduction to the physics of galaxies, stars, the interstellar and intergalactic medium in the early Universe.

The exploration of the first billion years of the history of the Universe, from the so-called dark ages to cosmic reionisation, represents one of the great challenges of contemporary astrophysics. During these phases the first structures start to grow forming the first stars, galaxies, and possibly also soon the first quasars. At the same time the dark, neutral Universe starts to be lit up and ionised by these sources, leading to its progressive reionisation anding at a redshift of about 6. Furthermore the first stars and supernovae begin to enrich their surroundings and the intergalactic medium, and to produce the first dust. In the recent years tremendous progress has been made on the theoretical understanding and on numerical simulations of the underlying astrophysical mechanisms. Furthermore, observations of signatures of reionisation and even direct observations of galaxies at redshifts larger than 6 are now becoming feasible.The observational and theoretical aspects of this story were comprehensively and pedagocically covered by the lecturers of the 36th Saas-Fee Advanced Course. This volume contains their worked out and updated lecture notes.

Abraham Loeb: First Light.- Andrea Ferrara: Cosmological Feedbacks from the First Stars.- Richard S. Ellis: Observations of the High Redshift Universe.