Signatures of Topological Defects in the Microwave Sky: an Introduction

An introduction to topological defects in cosmology is given. We discuss their possible relevance for structure formation. Especial emphasis is given on the signature of topological defects in the spectrum of anisotropies in the cosmic microwave background. We present simple analytic estimates for the CMB spectrum on large and intermediate scales and compare them with the corresponding approximations for models where initial perturbations are generated during an inflationary epoch.