Open-top chambers (OTCs) have been invaluable for ecologically-relevant studies of plant responses to atmospheric and climatic change. Earlier experiments with potted plants in controlled-environment chambers clearly demonstrated many of the biochemical and physiological responses to elevated atmospheric CO₂, but to address longer-term questions related to the global carbon cycle, it was critical to ask whether those responses would be sustained over several growing seasons under field conditions of multiple, fluctuating and interacting environmental variables. OTCs permitted research with young trees with unconstrained root systems in native soil for up to four growing seasons. The OTC experiments showed that short-term responses in controlled environments were sustained under field conditions, and the results could be used to inform models and make inferences about longer-term, ecologically-relevant responses. Modification of OTC design also has permitted studies of plant responses to climatic warming, which is especially difficult in other exposure systems. As valuable as these CO₂ and warming studies have been, in OTC experiments with trees, structure-function relationships are confounded with plant ontogeny, and critical stand-level responses cannot be addressed. Although more difficult and expensive to implement, free-air CO₂ enrichment (FACE) studies became necessary. FACE studies have been better designed and more valuable because they were developed around testable hypotheses that first emerged from OTC experiments. Experimental manipulation of whole ecosystems to elevated CO₂ and warming continue in OTCs, but they are restricted to small-stature ecosystems. For example, the Old-Field Community Climate and Atmosphere Manipulation (OCCAM) experiment currently is investigating the interactive effects of elevated CO₂, warming, and altered soil moisture in a constructed community of seven old-field species, focusing on how changes in community composition alter the relationships between aboveground and belowground response. This experiment, which fulfills the mandate for multi-climate-factor ecosystem-response studies, would be impossible without open-top chambers.