Influenza characteristically returns. An influenza period usually comprises from three to five years, with one or two very mild epidemics at the beginning which may frequently be overlooked, then of wide pandemic spread, to be followed by endemic recurrences for as long as two or three years. During these influenza periods the intervals between waves are frequently so nearly equal or multiples of each other as to force the question of a periodic law. Not only thus, but even on a larger scale does the disease appear with a certain uniform regularity. The great epidemics are separated frequently by intervals approximating decades. Stallybrass calls attention to the epidemic years in England, which are 1789–90, 1802–03, 1830–32, 1840–41, 1848–49 and 1851, 1854, 1869–70, 1879, 1890–91, 1898, and 1918–19. With the exception of 1854 all of these dates are around the end of a decennium. Yet, again, in the successive waves of an individual epidemic, as has been pointed out by Pearl, there is very roughly some periodicity. Are these admittedly obvious phenomena fundamental features of the life cycle of the influenza virus, or are they incidental, due to extrinsic causes, changes in the pabulum, in the host as an individual, or in the host as a community, or changes in climatic conditions? Is it a basic feature upon which we must build our conception of the epidemiology of epidemic influenza, or is it more a feature of chance? The evidence to date is conflicting and incomplete. The answer lies in the future. Periodicity in the acute infections is not a new subject. It has been discussed in various other diseases, particularly in measles. For many years epidemiologists in many parts of the world have reported the observation of a periodicity in epidemics of measles. It is generally regarded as an established fact that each locality suffers from epidemic waves of this disease and that the period is somewhere about two years. In certain relatively small localities in England where registration statistics have been kept for many years the Health Officers count on an epidemic every two years. In some places the epidemic is expected to fall during the even years, while in others it occurs in the odd years. Brownlee has been one of the foremost investigators in the periodicity of influenza, but since his communication on that subject was very brief, we take occasion to quote first from his article on the periodicity of measles, thereby gaining a more comprehensive knowledge of his theory, and at the same time becoming able to compare the periodicity in the two diseases.