This chapter focuses on assessing and managing external nutrient loads to a waterbody. To prevent cyanobacterial blooms, it is therefore essential to introduce sewerage together with introducing appropriate wastewater treatment, including sufficient nutrient removal, to avoid exceeding critical loads to the receiving waterbody. Estimating nutrient loads from large catchments with a range of possible nutrient sources can be particularly challenging. Nutrient budgets can be used to determine areas at risk of releasing nutrients to waterbodies, to identify driving factors behind nutrient pollution resulting from agriculture and to follow trends over time. Effective treatment technology is available to remove nutrients from effluents, and success can be readily demonstrated by comparing upstream and downstream nutrient concentrations in the receiving waterbody. The current load then needs to be differentiated according to the locally relevant sources and pathways in order to identify measures for reducing or controlling loads from these sources.