Many in the field of early literacy development and learning believe strongly that play and literacy share common ground, but they have found the idea difficult to prove. While some primary research indicates a positive relationship, the impact of play seems to occur at different levels of development, which complicates how researchers view its influence on early literacy policy and practice. The authors use a critical-appraisal process, more common in the medical field, to describe the best available evidence from a corpus of play-literacy studies. Appraising some seventeen studies and their levels of evidence, strength of design, and "worth to practice" findings, they arrive at a description of three major domains in the play-literacy relationship. They assert that the better evidence in each domain shows the effects of play on literacy skills while revealing the research yet needed to demonstrate the relevance of each domain for early literacy. They recommend the further use of the critical-appraisal process in the play-literacy field to build a body of high level evidence that will have a major impact on early literacy practice. (Contains 2 figures.)