Knowledge Portals (KPs) are highly integrative Knowledge Management Systems (KMSs) that promise to synthesize widely dispersed knowledge and to interconnect individuals in order to provide a 'one-stop knowledge shop'. Yet, KPs face major challenges in practice, as the intricacies of knowledge exchange are subject to varied individual and social factors. At the same time, growing anecdotal evidence from case studies indicates KPs' enormous potential. In this paper, we take some initial steps towards a theory for KPs that more distinctly conceptualizes KPs and emphasizes a KP's role to unify networking and repository KMS features. We describe three major challenges to successful KP deployment: (1) sufficient contribution, (2) favorable organizational culture, and (3) knowledge integration-and validate these as applicable to KPs through a review of 42 empirical papers.