People have been interested in cosmological questions since ancient times. What is the universe made of? How and why did it begin? Will it ever end? Over the past century – and especially over the past few decades – new ideas in theoretical physics have combined with new types of telescopes and astronomical observations, allowing us to make dramatic progress in finding answers while uncovering new questions. From the mysterious "dark matter" and "dark energy" that appear to dominate the cosmos, to the nature of the big bang singularity, and the "initial conditions" of the universe, cosmologists at Perimeter are involved in a range of theoretical and observational projects to advance our understanding of the universe. Cosmology is intimately related to other branches of research at Perimeter, including particle physics, quantum fields and strings, and strong gravity.