Chapter 14 discusses usability in a world of interactive devices. Interaction must be based on a clear and consistent model of the task; language must be designed for ease of comprehension; system states must be visible to users at all times; the operation of automation must be transparent to users; modes must be clearly defined and explicit with clear indication of mode changes as they occur; and users must be able to define the level of automation in any task. Mental models are introduced by examining recent research on users' models of the World Wide Web. Principles for the design of user-friendly language are given and basic applications are described together with examples of how to calculate readability metrics for written communications. Interaction with virtual environments (VEs) is discussed and recent research on self-driving vehicles is reviewed against a more general consideration of trust in automation. Methods for designing summative scales for usability assessment are given and there is a broad introduction to psychometrics. Worked examples of the use of statistical techniques are given, including Cohen's kappa, Cronbach's alpha, and the calculation of limits of agreement for examining test–retest reliability. Guidance for the conduct of focus groups is provided.