John Dalton was the first person to systematically explore colour blindness (more appropriately called colour deficiency since the vast majority of people with this condition can see colour but have poor colour discrimination. He presented a famous paper - Extraordinary facts about colour vision - in Manchester in the 1790s. We now know that whilst most of us are trichromats (having three types of cones in our retina) some people are dichromats (with only two cone classes) or anomalous trichromats. Recently we have discovered that some people are even tetrachromats and might have exception colour discrimination. This podcast is all about colour blindness.