Lithium depletion in the Universe remains a matter of investigation This book aims to carry out a wide-ranging inquisitive review of over half a century of astronomical observations of lithium and astrophysical developments to understand its behaviour in diverse environments across the Universe. The scope is wide, from the primordial nucleosynthesis in the early universe, to the evolution in the Milky Way and other galaxies and the internal processes that determine the depletion or preservation in stars, brown dwarfs and exoplanets. The approach is to focus on the basic concepts, to describe the chronology of key observations and theoretical developments, with as little mathematical equations as possible, and to provide critical assessment of ongoing controversies and unsolved problems.