There is a lot of difference in the food requirements of the various butterfly species. Butterflies that have to rely upon one particular plant species are called monophagous. Thus, the caterpillars of the Small Tortoishell, only feed on Common Nettles and those of the Dusky Large Blue solely on the Great Burnet. Oligophagous species, like the Small Copper, feed on more than one species within one genus, in this case Common Sorrel and Sheep's Sorrel. The polyphagous species use a wide range of plants from diverse genera for their food. The caterpillars of the Holly Blue can be seen on buckthorn, dogwood, spindle-tree, ivy, holly, heathers, and many other shrubs and herbaceous plants.

In most cases, the female butterfly’s choice of food plant seems to be decided by the condition of the plant, or by where the plant is growing. This applies especially for species where eggs are laid in the vicinity of the larval food plant.

The caterpillar is the gobble-and-grow stage of the butterflies. The outer skin or cuticle of the caterpillar does not grow, and is replaced by a new one by moulting four to six times before the caterpillar is fully-grown. The amount of food that a caterpillar needs depends on its size and growth-rate. Besides that, the quality of the food plays a role; caterpillars that grow quickly, such as the Large White, mostly feed on more nutritious plants than slow-growing caterpillars such those of the Silver-spotted Skipper.

Looking at the adult butterflies, it is clear that the food or energy requirements differ a lot between species. Butterflies that emerge from the chrysalis with their eggs already developed, or with a large amount of fatty tissue, need little or no food. This applies for the Small Heath, that can live on vegetation poor in flowers. However, most species need a lot of food or energy in order to develop their eggs. Most species get this energy from nectar, as, for example, the Brimstone, Small Tortoiseshell, and Common Blue. There are other species, that although they need food, seldom visit flowers. Instead, they live on honeydew, a sugary secretion produced by aphids, or from sugars that some trees exude from their leaves or buds, or from wounds in the branches or trees. The Purple Hairstreak and Brown Hairstreak feed on honeydew, and the Camberwell Beauty on sap oozing from trees.
The males, in turn, may need certain minerals for the development of sperm, and can be seen on the ground, feeding from carrion or dung. (006.MOV, Limenitis populi) The males of the Poplar Admiral can be seen doing this.