How can political challengers avoid co-optation and other forms of moderation? This dissertation illuminates how institutional participation led to the co-optation of the Indigenous Australian movement and the factors which equipped the Ecuadorian Indigena movement to elude a similar fate. Most contestants in political struggles must at some point consider participation or co-operation with an opponent. This dissertation's findings suggest how activists might evaluate such arrangements, and, if they select engagement, how to proceed with full awareness of the advantages and disadvantages. Unlike many other scholars, I differentiate productive and counterproductive institutional participation: productive participation results in concessions without unduly weakening the movement whereas counterproductive participation jeopardizes challenger influence in exchange for paltry gains. Based on meticulous research---including interviews and archival data---and using process tracing methodology, my analyses suggest that the Indigena movement resisted counterproductive participation, thereby avoiding co-optation, because numerous activists had developed an advanced consciousness: a cognizance of the mechanisms underlying elite domination and an awareness of the need for countervailing movement autonomy and safeguards. By maintaining a powerful organization, they could also constrain potentially deleterious participation by less aware activists. In contrast, the Indigenous Australian movement's lack of widespread consciousness and weak organizational capacity contributed to its co-optation and demobilization. In addition to suggesting how activists can avoid co-optation and benefit from participation, my research explores how they may acquire an advanced consciousness. This study also has implications for the agency-structure debate: Is challenger behavior shaped by the challengers themselves or the external environment? Critics of the political process theory synthesis---an amalgamation of resource mobilization theory, the political opportunity structures perspective, and framing analysis---claim that it privileges structural over ideational factors. In assessing the synthesis' ability to explain the cases, I find it elucidates why one movement succumbed to structural impulses, but cannot explain the other's resistance: the synthesis model indeed fails to account for agency, consciousness, and related phenomena. Although recent scholarship has attempted to "bring culture back in," the underlying theorizing is often narrow and empirically weak. This dissertation overcomes such shortcomings by offering theoretical insights grounded in real-world data.