Background: Recent curriculum studies raise a number of questions concerning teachers' implementation fidelity, including the extent to which fidelity to multiple curriculum components is achieved and measured and the extent to which fidelity serves as a mechanism for impacting children's learning. Objective: Within the context of a language and literacy curriculum supplement designed for use at scale, we investigated (1) teachers' fidelity across the multiple dimensions identified in the literature (e.g., Dane and Schneider in "Clin Psychol Rev" 18(1):23-45, 1998) and interrelations among these dimensions and (2) associations between measures of fidelity and the language and literacy gains made by children. Method: We examined the fidelity of 74 preschool teachers implementing" Read It Again!". Multiple measures of adherence, exposure, quality of delivery, and participant responsiveness were collected across the year of implementation, and children's (n = 295) language and literacy gains were directly measured. Results: Descriptive statistics demonstrated generally high implementation fidelity across all dimensions. Correlational analyses showed few interrelations among fidelity measures and few associations with child gains. Conclusions: Findings suggest that teachers can exhibit fidelity to multi-componential language and literacy curricula designed for wide-scale use. Findings also support fidelity as a multidimensional construct and suggest that researchers utilize multiple measures to capture both within- and between-teacher variation in fidelity, while also pursuing additional studies to better understand the measurement and functioning of fidelity to inform future work.