Our understanding of the universe has been revolutionized by observations of the cosmic microwave background, the large-scale structure of the universe, and distant supernovae. These studies have shown that we are living in a strange universe: 96% of the present day energy density of the universe is dominated by so-called dark matter and dark energy. But we still do not know what dark matter and dark energy actually are.This book presents lectures from the 186th Course in the Enrico Fermi International School of Physics entitled New Horizons for Observational Cosmology, held in Varenna, Italy,