This chapter examines cognition as a notion critical to understanding assessment and to developing effective assessment instruments and practices. The discussion uses a perspective that combines mainly knowledge from the fields of cognitive science and sociocultural theory. The perspective of cognitive science allows examination of processes such as understanding, learning, reasoning, and problem solving. Sociocultural theory allows examination of how those cognitive processes are shaped by culture and society. The chapter provides the following sections: Basic Notions of Cognition, Cognition and Culture, and Assessment Activities with Focus on Cognition. Basic Notions of Cognition discusses concepts originated from the field of cognitive science. Cognition and Culture discusses how culture shapes thinking process and how culture and language shape the ways in which students interpret tests and respond to them. Assessment Activities with Focus on Cognition explains how assessment can be used as a tool for probing understanding and informing instructional decisions beyond simply assigning grades.