The effects of variations in light intensity, photoperiod, and light quality on shoot dry weight, tillering, and leaf growth of barley (cv. Prior) have been examined in controlled environments. The rates of tillering and of dry matter production were primarily dependent upon the total radiant energy incident upon the plants. Tillering was unaffected by changes in the photoperiod (independent of light energy) or in the spectral composition of the light which profoundly affected apical development. At low light intensities, tiller buds on the main axis only elongated, whereas at high intensities secondary and higher-order tillers were produced. The largest number of tillers was associated with the coleoptile node, and tiller production declined regularly with each successive node up the main axis. The rate of leaf emergence on the main axis was relatively insensitive to changes in light intensity. At higher intensities there was an accumulation of expanding but unemerged leaves as the formation of leaf primordia progressively outstripped the rate of leaf emergence. Mature leaf size and shape were determined by the interaction of two control systems a heteroblastic change from node to node linked with apical development and hence indirectly affected by the spectral composition of the light source; and a direct effect of light intensity in the short photoperiod, probably mediated through carbohydrate supply.