This thesis consists of two parts. Part 1 is devoted to the study of nonstable K1 of quadratic groups using Bak's dimension theory. Part 2 is devoted to the study of the reduced K-theory of central simple algebras and is based on three papers published in 2001/2002: (1) Wedderburn's factorization theorem, (2) On central series of the multiplicative group of division rings, (3) SK1-like functors for division algebras.