Putting on a pair of polarised sunglasses is as close as most of us get to seeing polarised light. Photographers also use polarising filters and, in both cases, the reason for placing such filters in front of eye or camera is to reduce glare and increase contrast within the image or scene viewed. Animal visual systems also utilise polarised light for these purposes, along with navigation, sexual signalling and detecting water. They rarely, if ever, use optical filters to achieve polarisation sensitivity; instead it is an intrinsic property of their photoreceptors. Linear polarising sensitivity is common in the animal kingdom, particularly in invertebrates such as arthropods (insects, crustaceans and spiders) and cephalopods. Linear polarising sensitivity is also known in vertebrates, including fish, birds and a few amphibians and reptiles. In truth, this ability is probably more widespread than we think, and in the cephalopods and many crustaceans it may replace colour vision. While circular polarising photography — used for cancer detection in medical imaging and for (explosive) mine detection underwater — might be considered an obscure man-made optical trick, some animals also have circular polarising sensitivity. Before going on to describe how and why animals utilise polarised light, we briefly examine what polarised light is, why it is called linear or circular, where it comes from and where it is frequently found in natural environments (Figure 1).