Reactive oxygen species (ROS) are unstable and highly reactive molecules naturally produced from cellular metabolism, especially in the mitochondria. ROS play essential roles in maintaining normal cell function, supporting immune defence and regulating cell signalling. However, ROS levels increase under stress, infection or injury, leading to oxidative stress caused by an imbalance of ROS production and neutralisation mechanisms. Oxidative stress can trigger inflammation, a biological response that aims to protect and the healing process. ROS help signal immune cells to the site of damage or infection, contributing to the inflammatory process. An extended period of elevated ROS levels will damage the essential cellular components, including DNA, proteins and membranes, leading to prolonged or chronic inflammation. This ongoing inflammation disrupts normal tissue function, leading to numerous health issues, including metabolic and degenerative diseases. This chapter introduces the basic concepts of ROS and inflammation, including how they form and interact. It provides a foundation of understanding for their dual roles in health and disease, setting the platform for deeper exploration in later chapters on their involvement in metabolic inflammation.