This dissertation compares the reproductive ecology of three species of intertidal barnacles in the context of their life histories. Emphasis is placed on the patterns of repro­duction shown by similar species exploiting different but overlapping ranges of the intertidal zone. The intertidal environment imposes stringent gradients of greater physical stress in the high range of tidal exposure and increasing biological complexity at the low range of tidal exposure. Selection for differing adaptive modes of reproduction, re­cruitment, growth, and survivorship may be expected in species occupying different positions along these environmental gradi­ents. Ideally, the problem is not just to measure how partic­ular aspects of these modes differ between species facing vary­ing degrees of environmental stress, competition, and preda­tion but it is also to examine how components of the reproduc­tive ecology of the species interact with important parameters of growth and demography throughout their life histories.