iLookOut, a web-based child abuse training for early childcare professionals (ECPs), has been shown to improve knowledge and attitudes related to correctly identifying and reporting suspected cases of child abuse. The overarching goal of the present study is to examine "what works for whom" for iLookOut in order to identify strategies for optimizing learner outcomes. This prospective study enrolled 12,705 ECPs who completed iLookOut (November 2014-December 2018). We used structural equation models to test whether learner demographic and professional characteristics were differentially associated with implementation outcomes (i.e., acceptability and appropriateness) and whether these mediated subsequent indicators of training effectiveness (i.e., gains in knowledge). Consistent with previous research, individuals with lower baseline knowledge scores showed greater knowledge gains (β = -.57; p < .001). Greater knowledge gains were seen for learners who reported higher acceptability (β = .08; p < .001) or appropriateness (β = .14; p < .001). Implementation outcomes strongly associated with knowledge gains included acceptability for female learners and appropriateness for learners who had not completed high school or had >15 years of experience in childcare settings. Where mediation was found, for the majority of groups, appropriateness emerged as the driving mediator. Implementation outcomes emerged as important drivers of knowledge change for most groups. The iLookOut Core Training's use of a multimedia learning environment, video-based storylines, and game-based techniques were endorsed by learners and correlated with increases in knowledge. Future work should explore why aspects of the iLookOut training are rated as less acceptable or appropriate by some groups and what changes would improve efficacy for low performing learners.