The book provides an introduction to the subject of mysticism, presenting it from the point of view of metaphysics, psychology, and symbolism. It examines the different stages of development a mystic typically experiences.

Underhill emphasizes the practical, here-and-now nature of spiritual life. She argues that spirituality is a genuine and abiding human fact, and that any complete description of human life must find room for the spiritual factor, and for the religious life in which it finds expression.