This paper provides a concise introduction to and evaluation of the Lake Taupo nitrogen cap and trade program established as part of Waikato Regional Council's recent Regional Plan Variation Five. The policy establishes a catchment-wide cap on nitrogen losses by allocating farmers individual nitrogen discharge allowances and allowing those farmers flexibility to trade allowances amongst themselves and to sell allowances to a public fund while remaining within the overall catchment cap. This paper seeks to explain the structure and evolution of the nitrogen trading market and to analyse its impact thus far by drawing on a wide variety of relevant perspectives. Research drawn from written material and basic quantitative data provide the basis for analysing the policy, while interviews with relevant stakeholders provide insight into the successful, surprising and contentious issues which arose throughout its development and implementation.