Reading Plesk and Greenhalgh (2001) ‘The challenge of Complexity in Health care‘ I began to think about the difference between the definition of Complex Adaptive Systems and Human Activity Systems. Is there a difference between the two? If there is no difference then why do we use to different terms? Perhaps one is to help differentiate that we are dealing with systems that involve human activity whereas Complex Adaptive Systems is a more generic term that can be applied to non-human activity such as the ecological world. If that is the case then why doesn’t the literature such as the one above refer to the systems as Human Activity Systems as Checkland has done in his publications.