Low energy neutron beams are used to address many questions in nuclear physics, particle physics and astrophysics. The scientific issues include elucidating the nature of time reversal noninvariance; understanding the origin of the baryon asymmetry in the Universe; describing the weak interaction between quarks and between nucleons; understanding the origin of the elements in stellar and big bang nucleosynthesis.This book summarizes how spallation neutron sources work and discuss the advantages of pulsed beams in reducing systematic errors in precision measurements. It also describes recent breakthroughs in ultracold neutron production, together with the physics that will be addressed by the new generation of intense neutron sources.