A Brief History of Our Understanding of BEC: From Bose to Beliaev

We review how our current ideas about BEC developed in the early period 1925-1965, which had the specific goal of understanding superfluid $^4$He. This history is presented by commenting on the key contributions made by Einstein, Fritz London, Tisza, Landau, Bogoliubov, Oliver Penrose and Feynman. We emphasize the emergence of the concept of a macroscopic wavefunction describing the condensate. Starting with the fundamental work of Beliaev in 1957, the period 1957-1965 was a golden era for theoretical studies of interacting Bose-condensed gases. This work provided a sound conceptual basis for understanding the properties of trapped atomic gases which were discovered thirty years later.