This chapter provides a general overview of Educational Testing Service's (ETS) development work for US K-12 English language proficiency (ELP) assessments, and focuses on how the Evidence Centered Design (ECD) framework is operationalized in the development of US K-12 ELP assessments. The chapter describes the design of task types and test items to assess young language learners' ELP in the current context of EL education in the US It delineates how ECD has been employed in ETS's K-12 ELP assessment development work, followed by a discussion of the construct for these assessments. The chapter provides a fairly detailed discussion of the design of task types, drawing examples from two US K-12 ELP assessment systems: the English Language Proficiency Assessment for the 21st Century (ELPA21) and the English Language Proficiency Assessments for California (ELPAC). It describes two task types in order to illustrate how ETS has designed test items to address the integrated nature of the standards.