Optical projection tomography (OPT) is a technique for three-dimensional (3D) imaging of small biological tissues. It relies on taking images of the specimen from many different angles, and then using computer software to recalculate the original 3D information. It is in many ways the optical equivalent of X-Ray Computed Tomography or the medical CT scan.

It is particularly useful for reconstructing vertebrate embryos and for examining the 3D anatomy of developing organs. The advantages of this technique over other 3D imaging methods: i) its ability to image at a higher resolution than magnetic resonance imaging (MRI), while at the same time being able to image specimens much larger than those possible using confocal laser-scanning microscopy. ii) Being an optical technique, OPT is also able to take advantage of the many coloured and fluorescent dyes which have been developed for tissue-specific or gene-specific staining.