In this chapter, the authors review three distinct theoretical discourses--educational measurement, hermeneutics, and sociocultural studies--to support the development of validity theory for the routine use of assessment by professionals working in complex, dynamic, and always partially unique educational environments. They review validity theories in educational measurement, paying particular attention to how the needs of and expectations for the local user of standardized assessments are considered in this literature. Next, they expand the conception of assessment, pointing to studies of assessment in use and to the implications for assessment in evolving conceptions of learning. Then, they turn to theoretical resources for validity theory that can be located within an interpretive approach to social science and that support the development of situated interpretations. They turn to hermeneutics and its potential for supporting/inquiring into the validity of interpretations, decisions, and actions in response to a range of educational issues, problems, or questions. They also turn to sociocultural studies and their potential for analyzing complex and dynamic learning environments and the interactions between them (such as when evidence and people cross boundaries between levels of the educational system). They use extended examples as transitions between sections to demonstrate the need for alternative theoretical resources and to illustrate the theoretical resources that precede or follow. The examples illuminate some of the issues involved in developing validity theory to support assessment (1) as it might be used "within" a focal learning environment (e.g., a classroom) by those faced with the decision of "what do we do next?"; (2) as it might be used "outside" the focal learning environment by others (e.g., administrators, other teachers, and policy makers) who are responsible for monitoring, supporting, or evaluating the focal learning environment; and (3) as it might be used by those who are seeking to learn about the design and support of learning environments and the assessment practices enacted within them. They close with an analytic overview of the sets of theories on which they have drawn and suggest next steps for research and practice. (Contains 1 figure and 48 notes.)