Daily totals of light from three sites in a deciduous wood near Cambridge, England are given for 1961-3. The seasonal pattern is light is compared with that in the open, and analyzed in detail in terms of the interaction of the latitudinal and climatic effects on light in the open with seasonal variation in canopy structure. Among the various features discussed, it is particularly emphasized that the "light" and "shade" phases, hitherto defined in terms of the relative amount of light in the wood compared with that in the open, do not coincide with the periods of high and low light as measured in absolute units. There is an absolute peak of light in the spring from March to May, the details being very much affected by the difference in the density of the canopy between summer and winter and by variations in the time of leaf expansion from year to year. In autumn, there is little or no increase in the light totals with leaf fall. It is emphasized that these relations might be very different in other communities, latitudes or climates. The light is very variable in quantity and quality from day to day. Whether taken over short or long periods, the total light in the wood is never a constant percentage or that in the open. It is necessary to consider diffuse and direct light separately when defining and analyzing light conditions. Some of the possible reasons for the particular features of the woodland light climate are discussed, together with their possible biological significance.