Abstract: An introduction to modern cosmology is given, focusing on theories for the origin of structure in the Universe. After a brief review of the theory of growth of cosmological perturbations and a summary of some important observational results, most of the article is devoted to discussions of the inflationary Universe scenario and of topological defect models of structure formation. The final chapter is a summary of the theory of cosmic microwave background temperature fluctuations and of the current observational status.