The main experimental approaches in aquatic community and ecosystem ecology will be presented along a gradient of scale: unenclosed field experiments, mesocosms with a natural mix of species, microcosms with an artificial mixture of species from cultures, and single species experiments in batch and continuous culture. Experimental manipulations usually consist of the addition or removal of (supposed) key organisms, addition of chemicals or alterations of the physical environment. Selecting the appropriate scale of experimentation is not only a question of technical and financial feasibility but also it has to consider the inevitable trade‐offs between realism and control. Conclusions will be more widely accepted, if they are supported by experiments at a variety of scales.

Key Concepts:

• The choice of the appropriate experimental scale has to face an inevitable trade‐off between realism and control.
  
• Unenclosed field manipulations have the highest degree of realism but the least degree of control.
  
• Mesocosms operate with natural species assemblages, permit replication and control of experimental manipulations but are limited in temporal scale (weeks).
  
• Microcosm experiments operate with artificial communities assembled from cultures, offer a high degree of control, but lack the pre‐history a mutual adaptation of the participating species.
  
• Experiments with single or few species rely on the ‘model organism’ concept.
  
• Small‐scale experiments are biased against the detection of slow and space requiring processes.