After a release of genetically modified organisms, monitoring of potential adverse effects on the environment is mandatory. The protocol used for monitoring should be previously tested in practical studies and must be standardised. Moreover, sampling methods and the evaluation of results must meet current scientific and technical standards. Due to their particular role in maintaining soil quality and in a multitude of ecological processes in agro-ecosystems, soil organisms belong to those groups for which VDI guidelines are being developed. The guideline 4331 Part 1 describes fundamental criteria for the selection and sampling of soil organisms for GMO monitoring and gives guidance for sampling design, sampling strategy and statistical evaluation. In the guideline three approaches are followed: (1) a compilation of previously known effects and exposure pathways, (2) a documentation of ecological functions of soil organisms (ecosystem services) as well as (3) a description of characteristic species compositions in the soil. The aim was to develop a selection matrix that helps to choose the appropriate animal groups to be sampled. Besides the habitat type and the ecological relevance, the selection matrix also considers the suitability of animal groups in terms of practical issues and, in specific cases, anticipated effects. Further parts of the guideline 4331 will describe sampling methods for relevant soil animal groups.