This paper uses results from research in Clinical Information Systems (CIS) design, the psychology of clinical reasoning, and Ecological Interface Design (EID) to motivate general design concepts for the human-computer interface of CIS for ambulatory care. There are three primary contributions of this paper to the fields of HCI and Healthcare Informatics. (1) EID is generalized to the very different complex domain of healthcare decision making, (2) The previous work in clinical domain modeling informs the interface design principles of EID, (3) EID holds together a top-down theoretical approach to interface design and a bottom-up approach from specific domains. A specific design for an implemented ambulatory care CIS for geriatrics is used to illustrate the generality of EID.