The increase in atmospheric carbon dioxide concentration [CO2] during the last century has been one of the causes of climatic global changes. In general, crops react to a greater availability of CO2with an increase in leaf photosynthesis, biomass and yield. In order to evaluate the effect of CO2 on crops, experimental trials need to be carried out using different approaches: closed (greenhouses, growth chambers, tunnels, closed top chambers), semi-open (open top chambers= OTCs) and open systems (Free-Air Carbon dioxide Enrichment facilities=FACEs). In this paper, a review of Open Top Chamber (OTCs) systems is presented as well as their uses, construction and operational details. OTCs are suitable tools for this type of experiment: they have the advantages of providing the crop with conditions that closely correspond to reality, can be placed in open field for several seasons and are easy to transport. However, the disadvantages are that inside the OTC, CO2 is lower than in the normal concentration of air and temperatures are higher than those found externally. Nevertheless, these systems are useful for this type of research, principally for crops with short growth cycles.