Chronological age is a marker in time but it fails to measure accurately the performance or behavioral characteristics of individuals. This paper models the complexity of aging by using a system model and a human function paradigm. These models help facilitate representation of older adults, integrate research agendas, and enhance remediative services and educational approaches. To facilitate addressing the complexity, a model of the driving process is presented which has three main components: the driving environment, the person, and the interface between them. The purpose is to use identified component processes to produce a representational system to: (1) identify critical combinations of factors that produce risks to the older driver; (2) study the interaction of mobility and aging processes; and (3) guide research strategies and analysis of experimental data. The paper presents the model and the first stages of integration of the functional activities used in driving. The methods for identifying the component activities and processes involved are discussed along with the problems faced in integration of component activities, definitions of risk, relation of aging process variables to activities, and the identification of expected performance of an individual. A summary discusses the application to direct services, licensure, educational programs, and other mobility activities. (Contains 139 references.) (Author/RJM)