The extant literature on implementation fidelity has found mixed evidence for empirically establishing the dimensionality of implementation fidelity. The current study aims to add to this growing body of literature by examining implementation fidelity in a book-reading intervention for young children's caregivers. Caregivers (n = 291) implemented Sit Together and Read 2 (STAR 2) with their preschool-age children. These data indicated that implementation fidelity was determined to be a four-dimensional construct including adherence/dose, quality of delivery, participant responsiveness, and program differentiation. The main findings of this work are twofold: (a) implementation fidelity is a more complex construct than some previous descriptions, and (b) early childhood education research should aim to report on all aspects of implementation fidelity.