The Integrated Water Resources Management (IWRM) paradigm, which underpin current water reforms in Tanzania focus on the use of statutory legal systems to regulate the use of water resources. However, Tanzania operates under a plural legal system, where the diverse customary systems are relied upon in the implementation of IWRM. Very few human activities are regulated by statutory laws alone. Neglect of customary laws may cause IWRM implementation to fail, or will have negative consequences for individuals and groups who were better served by customary-based systems. This paper describes statutory and customary systems of managing water resources and discusses some of the challenges of implementing IWRM whilst taking appropriate account of customary laws in Tanzania, with the Rufiji River Basin as a case study.