Macular oedema is associated with several conditions that lead to blindness. Accurate measurement of macular thickness is important in order to follow disease progression and evaluate treatments. Four techniques are examined to determine the best reference standard for the detection and quantification of macular oedema: ultrasound, optical coherence tomography, the retinal thickness analyser, and the scanning laser ophthalmoscope. The three optical techniques have the highest resolution and sensitivity, in particular optical coherence tomography. Ultrasound can be useful where dense opacities preclude optical imaging.