Scholarship on learning progressions (LPs) in science has emerged over the past 5 years, with the first comprehensive descriptions of LPs, on the nature of matter and evolution, published as commissioned reports (Catley, Lehrer, & Reiser, 2005; Smith, Wiser, Anderson, & Krajcik, 2006). Several recent policy reports have advocated for the use of LPs as a means of aligning standards, curriculum, and assessment (National Research Council [NRC], 2005, 2007). In some ways, LPs are not a new idea; developmental psychologists have long been examining the development of childrens' ideas over time in several scientific domains. However, the emerging research offers renewed interest, a new perspective, and potentially new applications for this construct. For these reasons, this special issue of the Journal for Research in Science Teaching is timely. Our goal in this introduction is to explain the motivation for developing LPs, propose a consensual definition of LPs, describe the ways in which these constructs are being developed and validated, and finally, discuss some of the unresolved questions regarding this emerging scholarship. © 2009 Wiley Periodicals, Inc. J Res Sci Teach 46: 606–609, 2009