Infinite-dimensional Clifford algebras and their Fock representations originated in the quantum physics of the free fermion field. This Tract begins with a definitive account of various Clifford algebras over a real Hilbert space. Chapter 2 contains a detailed account of creators, annihilators, Fock representations and parity. Transformation properties of Fock representation under Bogoliubov automorphisms are discussed in chapter 3: this leads to the restricted orthogonal group. In the final chapter the authors discuss inner Bogoliubov automorphisms and construct infinite-dimensional spin groups. Apart from assuming a basic background in functional analysis and operator algebras, the presentation is self-contained with complete proofs, many of which offer a fresh perspective on the subject. The book will therefore appeal to a wide audience of graduate students and researchers in mathematics and mathematical physics.