With one exception, these papers are original and fully refereed research articles on various applications of category theory to algebraic topology, logic and computer science. The exception is a survey paper which gives an account of classical Tannaka duality in such a way as to be accessible to the general mathematical reader. No expertise in either representation theory or category theory is assumed. Topics such as the Fourier co-transform, Tannaka duality for homogeneous spaces, braided tensor categories, Yang-Baxter operators, Knot invariants and quantum groups are introduced and studied.