Environmental concerns and depleting phosphorus reserves have led to intensified research efforts to close nutrient cycles by recycling of nutrient-rich human wastes in the last decade. Treated wastes for crop nutrition need to contain plant-available nutrients and be free of, or at least low in, potential contaminants. This chapter presents the key issues in the optimum use of treated wastes in crop nutrition. The chapter discusses technical processes such as precipitation of salts, incineration and post-treatment of ashes, as well as production of carbonaceous materials. The chapter characterizes the resulting materials regarding their plant availability and possible inorganic and organic contamination. Finally, the chapter looks ahead to future research objectives for the optimized use of treated wastes.