Determining the nature of our Universe and its constituents is one of the grandest and most fundamental questions in modern science and drives the research underlying many of the chapters in this book. The newest puzzle is the observed acceleration in the rate at which the Universe is expanding. The knowledge that the Universe is not a static and unchanging place, but is growing with time, was attained in the beginning of the 20th century. The early work of Slipher, Hubble, and Humason (Slipher, 1917; Hubble, 1929; Hubble and Humason, 1931) showed that nearby “spiral nebulae” are receding from the Earth in every direction on the sky at velocities proportional to their inferred distance. The only viable interpretation of these observations is that the Universe is getting larger over time, expanding in every direction.