This meta-analysis pools data from 25 years of research on the order of acquisition of English grammatical morphemes by students of English as a second language (ESL). Some researchers have posited a "natural" order of acquisition common to all ESL learners, but no single cause has been shown for this phenomenon. Our study investigated whether a combination of 5 determinants perceptual salience, semantic complexity, morphophonological regularity, syntactic category, and frequency accounts for the variance in acquisition order. Oral production data from 12 studies, together involving 924 participants, were pooled to obtain weighted accuracy scores for each of 6 grammatical functors. Results of a multiple-regression analysis showed that a large portion of the total variance in acquisition order was explained by the combination of the 5 determinants. Several of these determinants, it was argued, can be seen as part of a broad conceptualization of salience. Since the article was originally published, a number of meta-analyses have appeared in the applied linguistics literature (e.g., Masgoret & Gardner, 2003; Norris & Ortega, 2000; Ortega, 2003), and a book on meta-analysis in applied linguistics research is forthcoming (Norris & Ortega), in press. Meanwhile, research on the acquisition of Hebrew as a second language has begun to provide cross-linguistic evidence for how different aspects of salience contribute to ease or difficulty of second language acquisition (DeKeyser, Alfi-Shabtay, Ravid, & Shi, 2005) and how salience interacts with age of learning (DeKeyser, Ravid, & AlfiShabtay, 2005).