This thesis examines Inwardness in the faith and practice of British Quakers. Inwardness is identified within the spiritual and mystical component of individual Friends’ experiences and discussed in terms of personal experiential knowing. Both academic and devotional discourses are used to clarify what is meant by ‘spiritual consciousness’, framed both within corporate, albeit mainly tacit, formulations of Inwardness, and expressed by leading exponents of Quakerism, at two different stages of the history of the Religious Society of Friends. The thesis makes an original contribution to scholarship in three ways: it identifies a distinct view of Quaker Inwardness in terms of process and state; it provides a new model of spiritual development through the Quaker worship practice; and it offers an explanation of spiritual maturity. The latter is identified with reference to an understanding of Interiority, which has consequences. Two Conditions and seven Elements of the process of gaining the state of Inwardness are identified and are found to be consistent between seventeenth and twenty-first century Quakers. Throughout the thesis analysis, reference to expansion of consciousness is interpreted in relation to mysticism, and proposes finally a new perspective on Quaker theology.