The pattern of production, development and survival of component tillers of spring barley (Hordeum distichum) cv. Triumph was investigated. Tiller appearance followed a well-defined pattern, with the emergence of the main shoot followed by that of primary tillers and by higher order tillers. The earliest emerged tillers survived the longest and contributed the largest percentage to grain yield; many of the higher order tillers died prematurely with those surviving contributing little to the grain yield of the whole plant. Tiller death and suppression of tiller production were coincident with the reproductive phase of main shoot development; some late tillering after anthesis was observed. The nutritional and hormonal control of tillering was investigated by applying a range of nutrient and plant growth regulator treatments. Pre-tillering applications of the growth retardant, Terpal, rapidly diminished the growth of the main shoot stem, leaves and roots whilst displaying a promotory effect on tillering. Terpal increased the production and early outgrowth of higher order tillers enabling a greater proportion of these to survive and produce ears; this did not result in any overall increase in yield as mean ear size was reduced. Another growth retardant, Cerone, and the auxin-antagonist, TIBA, similarly promoted tillering. The control of apical dominance systems in uniculm and tillering varieties of barley and wheat were compared. Tiller buds were revealed in the leaf axils of young uniculm plants and in barley it was impossible to initiate their outgrowth whereas several treatments, notably Terpal, Cerone and TIBA promoted tiller production in the uniculm wheat plants. These results are discussed with respect to competition for assimilates/nutrients and the roles of endogenous ethylene, auxin, gibberellins and cytokinins. The possibility of using growth regulators to modify tillering on a commercial scale is discussed and the prospect of seed treatment as a useful method of growth regulator application is also considered.