The neutron is an elementary particle that has been extensively studied, both theoretically and experimentally. This book reviews and analyses the results of the mainly experimental research on the neutron and rationalizes what is known so far about its intrinsic properties. The book covers topics that have not previously been dealt with in detail, including the gravitational properties of the neutron, precise determination of its mass, beta-decay, and its electromagnetic properties. This translation is an updated version of the original Russian text and also covers the more recent advances made during the past 7-8 years, including the application of methods based on the storage of ultra-cold neutrons to the study of beta-decay, new precise measurement of the mass of the neutron, and confirmation of modern theories of the internal structure of the neutron.