The yield of crop dry matter is closely dependent upon the amount of solar radiation intercepted by the crop canopy. This in turn is primarily determined by the amount of leaf area and its persistence. This study was concerned with the influence of environments, nitrogen supply and plant density in controlling apical development and leaf growth and development in barley. Three series of experiments were carried out on sequentially sown spring barley (cv. Claret) to cover the whole range of the natural variation in environmental variables. In the first series of experiments there were 4 sowing dates, 4 levels of nitrogen and plants were grown in perlite in small pots. In the second series there were 3 sowing dates, 4 levels of nitrogen and plants were grown in soil and sand compost in small pots. In the third series of experiments there were 3 plant densities, 2 levels of nitrogen and plants were grown in soil and sand compost in large tanks. A strong effect of growth media and size of pot on leaf growth was observed. The plants grown in soil had longer leaves and had more tillers than plants grown in perlite. Leaves were even longer when plants were grown in large tanks. Primordia initiation on the main shoot apex, leaf appearance and leaf extension were best described as linear function of thermal time rather than Julian time. Rate of leaf appearance on the main shoot was found to be linearly related to the rate of change of daylength at crop emergence. Final leaf length depended upon both the rate and duration of leaf extension. However, most of the variation in final leaf length was due mainly to variation in leaf extension rate. Leaf extension rate increased with nitrogen supply. A significant quadratic relationship between leaf extension rate and leaf nitrogen content was observed. It is suggested that irrespective of growing conditions leaf extension rate (in mm °Cd-1 ) is most probably controlled by the nitrogen content in the leaf rather than external nitrogen supply. High temperatures, long days and fast leaf appearance rates all resulted in shorter leaf extension duration. Of these variables variation in temperature accounted for the greatest proportion of variation in leaf extension duration. In general all the plant parameters recorded were affected by nitrogen supply, but the effect was more pronounced in perlite. There was a smaller response to applied nitrogen in soil because of the residual nitrogen supplied by the breakdown of organic matter. Lamina area and dry weight increased with the position of leaf on the main shoot up to 2 leaf insertions before the flag leaf. The flag leaf was always much smaller than the subtending leaves. This ontogenetic drift in leaf size was associated with variations in leaf extension rate and leaf extension duration of the leaves. Final leaf size was affected by plant density. As density increased the size of the first three leaves was increased but the size of upper leaves was dramatically decreased. As density increased, final leaf number and the position of the largest leaf on the main shoot were decreased. Nitrogen affected the position of the largest leaf on the main shoot. As nitrogen supply increased the position of longest leaf moved higher up the main stem. This pattern was also modified by sowing date. In sowings made in June, where rate of crop development was fastest, leaf 4 was the first leaf to show response to nitrogen. In sowings made in September, which developed more slowly, leaf 6 was the first leaf to show response to nitrogen. These effects are attributed to effects of internal competition for nitrogen. This suggests that the size of the later leaves is reduced due to lower availability of nitrogen. Early stem extension will also result in greater competition for nitrogen. On this basis one would expect a large response to nitrogen in fast developing crops and this was the pattern observed in these experiments. For most of the leaf growth parameters recorded in these experiments there were significant sowing date * nitrogen supply * leaf position interactions, which have not been reported in previously published investigations. This indicates the complex way in which these factors control leaf growth.