As we have seen in the previous chapters, humans continuously delegate and distribute cognitive functions to the environment to lessen their limits. They create models, representations and other various mediating structures, that are thought to be aid for thinking. The aim of this chapter is to shed light on these design activities. In the first part of the chapter I will argue that these design activities are closely related to the process of niche construction. I will point out that in building various mediating structures, such as models or representations, humans alter the environment and thus create cognitive niches. In this sense, I argue that a cognitive niche emerges from a network of continuous interplays between individuals and the environment, in which people alter and modify the environment by mimetically externalizing fleeting thoughts, private ideas, etc., into external supports. Cognitive niche constructionmay also contribute to making available a great portion of knowledge that would otherwise remain unexpressed or unreachable. This can in turn be useful in all those situations that require the transmission and sharing of knowledge, information and, more generally, of cognitive resources.