For organizations to survive and flourish in a changing environment, their business processes need to be flexible. Ashby’s law of requisite variety applied to business processes postulates that a robust process needs to exhibit as much variety in its responses as there are stimuli in its environment. An essential aspect of the design of a business process is to know to what stimuli it needs to respond to. In this paper, stimuli to the requirements of business process flexibility are examined. A classification framework is proposed to characterize the environmental stimuli. Using this framework, business process designers can visualize variations of the stimuli affecting the process. They can then consider if the business process support system would adequately manage the variations in the stimuli. The use of the framework is illustrated with the example of a university train ticket purchasing process.