In the context of further developing the excellence of its scientific assessments, enhancing their transparency and openness and in line with the trend in the scientific community, EFSA started the PROMETHEUS project (Promoting Methods for evidence use in scientific assessments) (2014–2016), which aims at further improving EFSA’s methods for “dealing with data and evidence” (i.e. for collecting/extracting, validating/appraising, analysing and integrating data and evidence) and increasing their consistency. The project envisages two scientific reports: the present document, which illustrates the principles and process for evidence use, and a second one, which will contain an analysis of the methods for dealing with evidence applied by the Authority and an impact analysis of the implementation of the PROMETHEUS process in EFSA. Methodological aspects related to the definition of the risk assessment process and domain-specific methodological issues are beyond the scope of PROMETHEUS. On the basis of EFSA core values, the principles for evidence use are: impartiality; excellence in scientific assessments (specifically related to the concept of methodological quality); transparency and openness; and responsiveness. The principles are interdependent and sometimes even in conflict; finding a compromise between them remains a responsibility of the assessors. A series of aspects to consider for promoting each principle is described. The process for dealing with data and evidence in a scientific assessment consists of: 1) Planning upfront a strategy for the assessment; 2) Conducting the assessment in line with the planned strategy and documenting the modifications to it; 3) Verifying the process; 4) Documenting and reporting the process, results and conclusions, and ensuring accessibility of methods and data. The principles and process illustrated apply to scientific assessments performed by generating data ex novo, using already existing data and to all types of scientific assessments irrespective of their objective, scope and authors. The principles also apply when scientific information is obtained by eliciting expert knowledge.