As humans, we continuously adapt our behavior to changes in our environment, and our cognitive abilities continuously develop over time. A major question for scientists has been to discover the (cognitive) mechanism that underlies the control of human behavior in real time, as well as cognitive development in the long term. This chapter will discuss two kinds of general approaches, which we shall refer to as the reductionist approach and the complex dynamic systems (CDS ) approach. Roughly speaking, the reductionist approach assumes that separate cognitive components, such as brain areas or processing mechanisms, are primarily responsible for behavior and development, by processing (and responding to) specific environmental cues. The CDS approach assumes that cognition and thereby the control of behavior and development are distributed over the brain, body, and environment, which continuously interact over time. The aim of this chapter is to compare the two approaches in terms of their assumptions, research strategies, and analyses. Furthermore, we will discuss the extent to which current research data in the cognitive domain can be explained by the two different approaches. Based on this review, we conclude that the CDS approach, which assumes a complex rather than a complicated model of cognition, provides the most plausible approach to cognition.