This thesis identifies the principal factors that drive the valuation of a law firm as a going concern, and integrates those factors into five forms of interrelating capital. An analysis of valuation theory demonstrates that the value of a law firm as a going concern is ultimately dependent on two dimensions: the volume of its economic income, and the perceived sustainability of that income. Valuation theory also assumes an entity to be valued that is, in some way, separate from the individuals who at any given time comprise it. This is potentially problematic in legal practice, where much of a firm's income and its sustainability may be dependent on the personal productivity and reputation of individuals. Consequently, the greater the degree of dependence on individuals, the lower the likely valuation. Social identity theory and organisation theory are applied to suggest how a firm can be recognised as a separate entity and achieve its own actual or perceived longevity. From a review of legal practice, the thesis derives fifteen factors that contribute to the volume of a law firm's economic income, and to the sustainability of that income and of the firm itself. These contributing factors are then shown to be, or more usually to be the outcomes of investments in, one of five forms of capital: financial, physical, human, social, and organisational. The salient features of each, and their contribution to valuation, are set out. Organisational capital, as the least developed concept theoretically, is defined and elaborated. In an application and development of resource-based theories of the firm, these five forms of capital, and the interrelationships between them, are then integrated into a capital approach to valuation.