This article addresses theoretical and empirical issues relevant for the development and evaluation of language learning progressions. The authors explore how learning progressions aligned with new content standards can form a central basis of efforts to describe the English language needed in school contexts for learning, instruction, and assessment. Learning progressions, in contrast with standards, can articulate incremental growth and are positioned to support teachers' formative assessment practices with K-12 students who are acquiring English as an additional language. Extending learning progressions that have taken hold in other areas of student learning to language learning and development requires conceptualizing (1) in what ways language progresses, and (2) how language growth can be supported by educators. The authors illustrate both considerations with the example of a new language learning progression and formative assessment initiative that are based on the characterization of explanations generated by students with a wide range of experiences with English.