The signaling principle recommends emphasizing relevant aspects of a multimedia message by means of signals (e.g., color coding). We determined the effectiveness of signals that highlight correspondences between text and pictures as well as possible boundary conditions by means of a meta-analysis. To this end, 58 potentially relevant articles were identified in a comprehensive search. After rating the studies based on inclusion criteria and correcting for biases, 27 studies were included in the meta-analysis yielding 45 pairwise comparisons with N = 2464 participants. Domain-specific prior knowledge, pacing of the materials, pictorial format, mapping requirements, and distinctiveness of signals were coded as moderators. For transfer and comprehension performance a positive small-to-medium effect size (r = .17, 95% CI [0.11, 0.22]) favoring signaled multimedia material was found, which was moderated by prior knowledge. The findings support the effectiveness of the signaling principle in particular for learners with low prior knowledge. (C) 2015 Elsevier Ltd. All rights reserved.