This chapter presents a collection of problems designed to work on the fundamental concepts and principles of computer security, as commonly introduced in introductory courses or foundational subject modules. The exercises are framed around diverse application scenarios to provide a practical understanding of key topics. Central to all exercises are the three core security properties—confidentiality, integrity, and availability—, which are analyzed through real-world examples and theoretical exercises. The chapter also introduces basic techniques for threat modelling and approaches to mitigation. Additionally, several problems delve into the fundamental security principles outlined in the seminal work of Saltzer and Schroeder [10], offering insights into design strategies that remain highly relevant in modern system security. Together, these exercises aim to foster critical thinking and a deeper comprehension of core security principles.