The presence of hypoxia, oxygen concentrations below normal physiological levels, is a pathophysiological condition found in tumours, ischemic tissue (cardiac ischemia, brain ischemia, etc.), inflamed tissue (e.g. rheumatoid arthritis), and vascular disease (e.g. diabetes). Given this, there is great interest in methods for the detection and imaging of hypoxia, particularly for research purposes. Analogues of validated hypoxia markers such as pimonidazole and EF5, bearing an azide or a terminal alkyne have been synthesised. These have been demonstrated to undergo a copper (I)-catalysed azide-alkyne 1,3-dipolar cycloaddition, referred to as ‘click chemistry’, with suitable probes. Preferred probes are fluorescent or fluorogenic molecules bearing a terminal alkyne or azide, respectively. This enables very sensitive and highly selective detection of hypoxic cells.