Competition between Elminius modestus (Darwin) and Balanus balanoides (L) is considered to be an important issue in the invasion of British shores by E. modestus and in explaining the coexistence of both species in the intertidal barnacle niche. Competition is discussed by reference to the concept of ecological performance, which is a term describing all adaptations and physiological and behavioural attributes of each species. Much information on the biology of cirripedes already exists, but the importance of predation and settlement behaviour in relation to competition have not been examined satisfactorily. Consequently these two ecological features are investigated in the thesis. The tolerance of developing embryos to extremes of temperature, and the infection frequencies of the castrating parasite Hemioniscus balani (Spence Bate) are also examined to assess their importance in the ecology of each species. All the available information is then summarised. Comparative assessments of ecological performance are made for each species and for each single feature of their ecology. The assessments are then analysed in an attempt to identify factors which are especially important in regulating competition between the two species. It is acknowledged that the approach is necessarily an over-simplification, but it is broadly concluded that biological mechanisms are more important than actions of the physical environment. Settlement behaviour and susceptibility to predation may be especially significant in influencing competition both during the initial colonisation by E. modestus and in the contest of its coexistence with B. balanoides.