The definition of system boundaries is an obligatory first step in every impact assessment. System boundaries represent the thematic and spatio-temporal frame within which an assessment is conducted. Impacts that occur outside of this frame are not considered.

The reason for setting system boundaries is threefold:

• First, potential users of the assessment results should be able to quickly find the information relevant to them. Having to search for this information amidst large amounts of other data would reduce the assessments usefulness.

• Second and more important, the number of effects and impacts that could possibly be investigated is too high to be handled in a single assessment. Only by restricting an impact assessment to a predefined frame, the method becomes operable.

• Third, the choice of relevant impact categories varies with the system boundaries. Adequate impact categories and respective indicators can only be identified with clearly defined boundaries.

Setting system boundaries should be done with great deliberation and properly documented. Decisions taken at this step will strongly affect the assessment’s results.

In particular, decisions are required on: