This dissertation focuses on Knowledge Intensive Business Services (KIBS). It aims to understand what these businesses are and to examine variety among them. In seeking to understand their diversity, I focus especially on the 'knowledge bases' at the core of their activities. The dissertation is based on three complementary studies. The first is a systematic review of the literature on KIBS. This is based primarily on a review of 130 carefully selected, relevant articles, and focuses on three questions: how are KIBS defined? how do KIBS compete? and how do KIBS innovate? The review shows that: (i) the literature is fragmented; most research does not build substantially on previous methods or findings; and (ii) while evidently heterogeneous, most of the literature has overlooked variety among KIBS. I also highlight what still needs to be known about KIBS.The second and third papers then focus on variety among KIBS, by classifying them according to their 'knowledge bases'. In the first of these papers, I classify KIBS according to their primary knowledge bases, following the SAS Model, which identifies three: 'analytical knowledge', 'synthetic knowledge' and 'symbolic knowledge'. Firms in three KIBS sectors: 'architecture and engineering consultancy'; 'specialist design'; and 'computer and IT services' are classified by their primary knowledge base according to information drawn from company websites. I then relate this classification to firm behaviour with respect to innovation, finding differences by primary knowledge base in the nature of the investments firms make to innovate, and in their propensities to innovate. In the second of the papers which relates 'knowledge bases' to KIBS, I develop the 'knowledge bases' approach conceptually, methodologically and empirically. Conceptually, I identify a hitherto unrecognised knowledge base: 'compliance knowledge'. This relates to the knowledge of, and to interpretations of, laws and regulations. This knowledge base does not fit with any of the existing SAS types. Methodologically, I extract fuller information from company websites, and develop more sophisticated approaches to measurement, which allows multiple knowledge bases to be present in any one firm. Empirically, I successfully identify 'compliance knowledge', alongside 'analytical' and 'symbolic knowledge'. I show that these are unevenly distributed across KIBS industries, including 'advertising and design', 'architecture', 'engineering consultancy' and 'market research', but importantly there is no one-to-one mapping between knowledge bases and industries. I discuss the implications of this, including for understanding the diversification of KIBS. This dissertation therefore contributes conceptually, methodologically and empirically to both understanding variety among KIBS and to the 'knowledge bases' literature.