In this tutorial we take a practical, evidence-based look at nitrous oxide—one of the oldest drugs in anaesthesia and still the only anaesthetic gas in routine clinical use. We explore its unique pharmacology, including the concentration effect, second gas effect and reverse second gas effect, explain why it remains useful during inhalational induction, and examine its true contribution to anaesthesia. The tutorial also reviews the evidence from major clinical trials, dispels common misconceptions, and discusses the important complications of nitrous oxide, including expansion of closed gas spaces, diffusion hypoxia, postoperative nausea and vomiting, vitamin B₁₂-related toxicity, and its environmental impact. By the end, you’ll understand when nitrous oxide adds real value to modern anaesthetic practice—and when it is better avoided.