The hypothesis of the chemical bond has had a long and valuable history in the development of chemical theory. According to classical views of valency, the hydrogen atom can sustain only one bond. During the early years of this century, to explain certain anomalies, the unorthodox notion crept into chemistry that hydrogen might sometimes accept a second, though weaker, bond; that it might become, in effect, bivalent. With the development of spectroscopic and crystal-diffraction methods, quantitative information about the 'hydrogen bond' began to accumulate; the concept became better defined and its validity well established. It proved to have an important effect on the behaviour of many systems, particularly those characteristic of living organisms.