The recognition of the branch of mathematics now called homotopy theory took place in the few years after the introduction of homotopy groups by
Witold Hurewicz in 1935. Since then, with numerous advances made by various workers, it has been playing an increasingly important role in the
expanding field of algebraic topology. However, there exists no textbook on the subject at any level except the extremely condensed Cambridge tract of P. J. Hilton entitled "An Introduction to Homotopy Theory."