The study of a life course cannot exist without the notion of time. But psychologists and sociologists don't take the notion of time seriously. One of the reasons why they tend to disregard time is that their desire is to seek a depiction that focuses on stability. Actually many of them might "find" the stable structure of personality and stable trail of life course as they construct it through data analyses that are blind to variability and dynamicity. Taking the equifinality principle into account is one of the breakthrough in describing the dynamics. The Trajectory Equifinality Model (TEM) which we describe in this chapter is a new methodological device for psychology. It is based on the systemic view and takes the notion of irreversible time seriously.