1. Observations in Scots (Pinus sylvestris) and Corsican (P. nigra) pine plantations of Thetford Chase, Norfolk, during five breeding seasons 1952-56, describe the food, food stock and feeding habits of great tits (Parus major), blue tits (P. caeruleus), coal tits (P. ater) and willow tits (P. montanus). 2. The density of tits breeding in nest-boxes is recorded. Coal tits, adapted to conifers, were much the commonest species; great and blue tits, adapted to broadleaved woods, bred commonly in the pine only where there were nest-boxes; willow tits were scarce and rarely bred in nest-boxes. 3. The food of the young was recorded by direct observation and identifications were checked from meals collected with an artificial nestling gape. Numerically, caterpillars formed about 50% of the food of early and late broods combined. Caterpillars from Scots pine important for early broods were Evetria spp. (Eucosmidae) from buds and shoots, Thera obeliscata (Hydriomenidae) and Ellopia prosapiaria (Selidosemidae) from pine needles; all of which were virtually absent from Corsican pine. For late broods, caterpillars of Panolis griseovariegata (Caradrinidae) from both Scots and Corsican pine were much the most important prey. 4. Typically, the caterpillar stock in pine was low (especially in Corsican pine) in April and May, increased slightly in June, and reached a peak in late summer. This contrasted with broadleaved woods, where the stock was greatest in May-June when early broods of tits were in the nest. 5. Great, blue and even coal tits nesting in the pine collected much food for early broods, but little for late broods, from scattered broadleaved trees up to 400 m from their nests; and those in Corsican pine plantations also foraged in distant Scots pine. Caterpillars from sources other than pine formed the largest group in the diets of early broods of great and blue tits, but were less important for coal and willow tits. 6. Proportions of different prey species fed to the young varied greatly each year. The diet of early broods of great tits was characterized by the absence of Evetria larvae. Spiders and insects other than Lepidoptera were taken more often by coal tits than by great or blue tits. Large caterpillars, notably Panolis, were much more prevalent in the diet of late than of early broods of great tits; but the diets of early and late broods of coal tits were broadly alike (too few late broods of blue tits for comparison). The diet of great tits was always more varied than that of the other species, but especially so in the pine; that of blue tits was broadly similar in pine and broadleaved woods. 7. All tits selected larger prey than those available at random in the pine. Great tits selected larger prey than did blue tits, and blue tits than coal tits; and even when feeding on the same prey species on the same dates and in the same plantations as coal tits, great tits invariably selected the larger specimens. 8. In the pine, great tits breed on dates and lay clutches appropriate to the broadleaved woods to which they are adapted; but whereas in broadleaved woods early broods do better than late broods, because food is more abundant, the reverse happens in the pine where feeding improves later in the season. In the pine, early broods of great tits were short of food, but late broods not. 9. Each member of a late brood of coal tits received much more food than did each of an early brood, but this was not reflected in their weights: presumably neither early nor late broods were short of food. Though adapted to conifers, coal tits are more productive breeding in broadleaved woods. They remain scarce in broadleaved woods probably because unsuccessful in interspecific competition for food in winter. 10. The numbers of caterpillars eaten represented only a few per cent of those present. One exception was in Scots pine from 1 May to 15 June 1956, when coal tits alone ate some 48 000 Evetria larvae or pupae per 10 ha (25 acres), representing about 20% of the stock. Despite the usually low percentage predation, intraspecific competition for food in summer may account for the modest decline in average clutch size with increasing density of birds. In summer, tits are unlikely to eat a large part of their food stocks or perceptibly influence the numbers of their prey, because each prey species is available for only a short period. Percentage predation is likely higher in winter, but on a different array of species. 11. Nest-boxes influence the local distribution of, especially, great and blue tits in pine in summer, but probably do not increase their overall populations--determined by food shortage in winter. Together, pine and broadleaved trees provide a sustained food supply throughout the summer. Hence selected broadleaved trees interspersed through the pine plantations might both enable more tits to survive the winter and improve their breeding; and so in turn enhance the general level of bird predation on forest insects.