In his groundbreaking paper, Tinbergen (1963, this volume) proposed that the four main problems in behavioral biology concerned causation, function (survival value), development (ontogeny) and evolution. Roughly speaking, causation and development are essentially causal or “how” questions, while function and evolution are functional or “what for” questions (cf. Hogan, 1994; Bolhuis and Macphail, 2001; Bateson, 2003; Bolhuis and Giraldeau, 2005). Functional or “what for” questions have also been called “why” questions (e.g., Bateson, 2003; Bolhuis and Giraldeau, 2005). This is confusing, as Tinbergen's four questions are also known as “the four whys”; for this reason I will use the term functional or “what for” questions. When Tinbergen wrote his paper, the prevailing emphasis in ethology was on causation. In his article he argued that more effort should be directed to the study of development, function, and evolution. Elsewhere in this volume, Crews and Groothuis and Hogan and Bolhuis discuss how developmental behavioral biologists took up Tinbergen's challenge. With regard to function and evolution, the behavioral ecology revolution in the 1970s was a somewhat delayed response to Tinbergen's clarion call. Recently, there have been increasing attempts to integrate the four questions. For example, in behavioral ecology there have been calls for a renewed research effort into the mechanisms of behavior (e.g., Krebs and Davies, 1997), and nowadays there is a field of research that is called – somewhat confusingly – functional ecology.