Secrets revealed only to those initiated; rituals which took place under the cover of night; promises of rewards to brighten a dark and grim afterlife- mystery cults were an important part of ancient religion, and yet their workings remain largely shrouded in secrecy today. This lecture will introduce what we know about mystery cults and their role in ancient religion, focusing on two of the most famous mystery religions of ancient Greece- the Eleusinian Mysteries in honour of the fertility and harvest goddesses Demeter and Persephone, and the Dionysian Mysteries in honour of Dionysos, god of wine.