This study begins with a discussion of the phenomenological experiences and teachings of mystics from both West and East, understood from within their own philosophical frameworks. The mystics are classified into four overlapping types: metaphysical mystics, devotional mystics, naturemystics, and occultist-mystics. Parallels and differences between the various types of experience are pointed out. The remainder of the study discusses certain philosophical points arising out of the phenomenological investigations, reference being made to the mystics studied to elucidate these points. The nature of mystical awareness and its episternological value are discussed, and the connections between mystical experience and the forms of expression used to convey it (symbolism, metaphysics, paradox) are elucidated. It is argued that mysticism is an experiential reality, equally as real as our other types of experience, to be understood in terms of its own philosophical structure. The role of Universals in mystical philosophy is discussed, and the bearing that this has upon methodologies used in philosophical evaluations of mysticism, and it is argued that mysticism should be seen from within its own, essentially idealistic, framework. Cross-cultural typologies of mysticism, and the essentialist/relativist debate, are examined, as well as the distinction between experience and interpretation. It is indicated that the question of the unity or diversity of different forms of mysticism must be considered on three levels: ontological source, experience, and interpretation. The parallels and differences between different categories of mysticism and different mystical traditions are discussed. It is suggested that a common spiritual life of humanity is shown by the unity of experience, symbolic expression, and so on, but that this does not necessarily involve an a priori "essence" of mysticism. It is concluded that we need to remain sensitive to both the unity and the diversity shown between different forms of mysticism.