The purpose of cosmology. Structure of this book. The observed universe. Existence of structure. Cosmic expansion. The age of the cosmos. Element abundances. Cosmic matter. Background radiation. Mass density. Isotropy and homogeneity. Evidence of evolution. Causal horizons. Cosmological equations. The dynamical equations. Explanation of Hubble's law. Expansion solutions. Redshift and Horizons. Theory of the redshift. Cosmological horizons. The density parameter. Cosmic flatness. The flatness approximation. Relation of density and horizons. Density parameter evolution. Cosmic thermal history. Primordial matter. The cosmic entropy density. How the universe cooled. The neutrino temperature. Decoupling and recombination. Element Synthesis. Time of element synthesis. The neutron to proton ratio. The helium mass fraction. Origin of matter. Existence of matter. Asymmetry as a physical theory. The generation of baryon number. Cosmological inflation. Cosmological problems. Cosmic matter fliuds. Chaotic inflation. The planck limit. Inflationary perturbations. The end of inflation. Dissipation of inhomogeneity. Eternal inflation. Comsic structure evolution. Matter as a fluid. Small perturbations. The Jeans Scale. Background radiation fluctuations. Dark Matter. Evidence of dark matter. The silk scale. Hot dark matter. Cold dark matter. Exotic Objects. Black holes. Magnetic monopoles. Boson stars. Survey of Theory. What have we learned? The present state of cosmological knowledge. Problems presently facing cosmology. Reprise. Relativistic cosmology. Journal articles. General physics.