The theory of nuclear β-decay has developed in two stages. The presentation of Pauli's neutrino hypothesis at the 1933 Solvay Conference initiated the first stage, and a period of renewed activity followed the discovery in 1957 that parity was not conserved in the process. Two distinct interactions, of polar vector and axial vector form respectively, are responsible for β-decay, and the neutron is itself the most suitable β-active nucleus with which to study the basic properties of these interactions. For this reason a continuing effort has been made to refine and improve the precision of measurements on the β-decay of the neutron. In this article the most important features of the phenomenon of β-radioactivity are summarized, the techniques employed to study the radioactivity and related properties of free neutrons are discussed in some detail, and the experimental results interpreted in terms of established theory in the field of weak interactions.