The assessment of computer and information literacy (CIL) is at the core of the International Computer and Information Literacy Study (ICILS). In this chapter we define and describe the CIL construct that underpins the assessment used in ICILS. Computer and information literacy was first defined and described for use in ICILS 2013, and is reviewed at the beginning of each new ICILS cycle, with reference to developments in CIL-related research, policies and curriculums, and with respect to its operationalization in previous ICILS cycles. In the chapter we trace the history of the CIL construct from its origins in the second half of last century through to its contemporary instantiation as a combination of technological proficiency with facets of information literacy and communication. In ICILS, the term “Computer and Information Literacy” underscores the significance of internet-based information search and evaluation within the broader competency of utilizing contemporary technology. We continue with an explanation of the process of deriving the CIL construct definition and structure from preexisting and preeminent definitions of related constructs. We then describe the structure of the CIL constructs and elaborate on the content of the four strands divided into eight aspects that comprise the CIL construct.