Psoriasis is a common skin condition that leads to patches of scaled skin which can be inflamed, sore and itchy. It can affect any area of skin on the body, as well as nails, and can be widespread and severe. Over the past 20 years, many new treatments have been developed and approved for severe psoriasis. Most of these newer treatments are given by injection and, as a group, are known as biologics. Many people respond well to biologics and have a meaningful improvement in their condition. There is a smaller proportion of people who do not respond well, or develop side effects, and so must switch drugs to try and improve their condition. In some cases, people need multiple lines of biologic treatment. Currently very little is understood about how well people respond when they are treated with three or more biologics in a row, as very few trials have been done in these cases. Our study aims to use data from people who have already been treated with biologics by their Dermatology teams in the NHS and use the information obtained during their normal clinic appointments to investigate this question. We will assess how well people respond to each line of biologic drug (1st to 10th). We will look at whether people respond as well or less well over time, the more biologics they have. People can be included in the research if they are 18 or over and have been treated with a biologic for psoriasis in participating hospitals in the NHS in England. In this study we are assessing data from events that have previously happened, and so no extra visits are required