In Celtic mythology, the Otherworld is the realm of the deities and possibly also the dead. In Gaelic and Brittonic myth it is usually a supernatural realm of everlasting youth, beauty, health, abundance and joy. It is described either as a parallel world that exists alongside our own, or as a heavenly land beyond the sea or under the earth. The Otherworld is usually elusive, but various mythical heroes visit it either through chance or after being invited by one of its residents. They often reach it by entering ancient burial mounds or caves, or by going under water or across the western sea. Sometimes, they suddenly find themselves in the Otherworld with the appearance of a magic mist, supernatural beings or unusual animals. An otherworldly woman may invite the hero into the Otherworld by offering an apple or a silver apple branch, or a ball of thread to follow as it unwinds.

Celtic mythology is the body of myths belonging to the Celtic peoples. Like other Iron Age Europeans, Celtic peoples followed a polytheistic religion, having many gods and goddesses. The mythologies of continental Celtic peoples, such as the Gauls and Celtiberians, did not survive their conquest by the Roman Empire, the loss of their Celtic languages and their subsequent conversion to Christianity. Only remnants are found in Greco-Roman sources and archaeology. Most surviving Celtic mythology belongs to the Insular Celtic peoples. They preserved some of their myths in oral lore, which were eventually written down by Christian scribes in the Middle Ages. Irish mythology has the largest written body of myths, followed by Welsh mythology.