This chapter from the course "Advanced Graphics & HCI" by Dr. Alan Blackwell at the University of Cambridge Computer Laboratory provides an overview of the field of Human-Computer Interaction (HCI). It emphasizes that the success of a software product is largely determined by how well it meets the needs and usability of its users. The course covers both theoretical and practical aspects of making software that is both intuitive and powerful. The introduction highlights the importance of understanding learning, memory, and problem-solving in designing usable software, which are typically studied in cognitive psychology. The course also discusses the role of engineering in creating efficient and effective products. HCI research involves experimental studies and design techniques, and the course includes case studies and online resources for further exploration. The chapter then delves into the historical development of user interfaces, from early command-line interfaces to modern graphical user interfaces (GUIs). It explains how the evolution of input devices has influenced interaction styles, such as algebraic languages, dialogues, WYSIWYG editors, menus, and direct manipulation. The chapter also discusses the principles of direct manipulation, which emphasize continuous visibility, physical actions, rapid and reversible operations, immediate feedback, and a modest set of commands. The course covers psychological user models, including models of visual input, physical output, memory, and problem-solving. It introduces the Model Human Processor, Keystroke-Level Modeling (KLM), and the Generalized Model of Problem Solving (GOMS). These models help predict user performance and guide the design of user interfaces. The chapter also discusses evaluation techniques for cognitive models, such as mental models, prototyping, and controlled experiments. It emphasizes the importance of user-oriented design methods, including prototyping, experimental studies, hypothesis testing, questionnaires, and cognitive walkthroughs. The goal is to understand user behavior and improve the usability of software products.This chapter from the course "Advanced Graphics & HCI" by Dr. Alan Blackwell at the University of Cambridge Computer Laboratory provides an overview of the field of Human-Computer Interaction (HCI). It emphasizes that the success of a software product is largely determined by how well it meets the needs and usability of its users. The course covers both theoretical and practical aspects of making software that is both intuitive and powerful. The introduction highlights the importance of understanding learning, memory, and problem-solving in designing usable software, which are typically studied in cognitive psychology. The course also discusses the role of engineering in creating efficient and effective products. HCI research involves experimental studies and design techniques, and the course includes case studies and online resources for further exploration. The chapter then delves into the historical development of user interfaces, from early command-line interfaces to modern graphical user interfaces (GUIs). It explains how the evolution of input devices has influenced interaction styles, such as algebraic languages, dialogues, WYSIWYG editors, menus, and direct manipulation. The chapter also discusses the principles of direct manipulation, which emphasize continuous visibility, physical actions, rapid and reversible operations, immediate feedback, and a modest set of commands. The course covers psychological user models, including models of visual input, physical output, memory, and problem-solving. It introduces the Model Human Processor, Keystroke-Level Modeling (KLM), and the Generalized Model of Problem Solving (GOMS). These models help predict user performance and guide the design of user interfaces. The chapter also discusses evaluation techniques for cognitive models, such as mental models, prototyping, and controlled experiments. It emphasizes the importance of user-oriented design methods, including prototyping, experimental studies, hypothesis testing, questionnaires, and cognitive walkthroughs. The goal is to understand user behavior and improve the usability of software products.