Different STIs are detected in different ways, and this chapter explains how. It outlines the main testing methods, urine tests, swabs and blood tests, and why the right one depends on whether an infection is bacterial, viral or parasitic. It then goes through common infections one by one, including chlamydia, gonorrhoea, syphilis, HIV, herpes, HPV, hepatitis B and C, and trichomoniasis, describing typical symptoms, the possible complications and how each is screened for. The chapter closes with pubic lice and scabies, which are diagnosed through a physical examination rather than a lab test, and are treated with topical medication.