40CFR Part 503 (1993), coupled with state regulations, govern biosolids recycling in the USA. The federal rule is risk-based (to protect against “reasonably anticipated adverse effects”), and assesses exposure to animals, humans, and the environment to biosolids-borne metals through 14 pathways. Limiting pathways (those identifying the smallest metal loadings that result in adverse effects) are used to calculate standards such as Ceiling Concentrations (Table 1 values Part 503) and Pollutant Concentrations (Table 3 values). The risk assessment is relatively straightforward, but data-intensive and frequently requires professional judgment to assess data quality and to fill scientific gaps. This requirement has attracted concern and criticism. Further, the risk assessment methodology has been characterized as out-dated in a recent NRC report. The 503 risk assessment process is described, with special attention to Mo, to demonstrate strengths and weaknesses and to demonstrate how standards can be derived. The use of common sense – not an official risk assessment component – is shown important to standards development and promulgation.