The concept of stigmergy has been used to analyze self-organizing activities in an ever-widening range of domains, including social insects, robotics, social media, and human society. Yet, it is still poorly understood, and as such its full power remains underappreciated. The present paper clarifies the issue by defining stigmergy as a mechanism of indirect coordination, in which the trace left by an action in a medium stimulates a subsequent action. It then analyses the fundamental concepts used in the definition: action, agent, medium, trace and coordination. Stigmergy enables complex, coordinated activity without any need for planning, control, communication, simultaneous presence, or even mutual awareness. This makes the concept applicable to a very broad variety of cases, from chemical reactions to individual cognition and Internet-supported collaboration in Wikipedia. The paper classifies different varieties of stigmergy according to general aspects (number of agents, scope, persistence, sematectonic vs. marker-based, and quantitative vs. qualitative), while emphasizing the fundamental continuity between these cases. This continuity can be understood from a non-linear dynamics that lets more complex forms of coordination evolve out of simpler ones. The paper concludes with two specifically human applications, cognition and cooperation, suggesting that without stigmergy these phenomena may never even have evolved. Past, present and future of the " stigmergy " concept The concept of stigmergy was introduced by the French entomologist Pierre-Paul Grassé (Grassé, 1959) to describe a mechanism of coordination used by insects. The principle is that work performed by an agent leaves a trace in the environment that stimulates the performance of subsequent work—by the same or other agents. This mediation via the environment ensures that tasks are executed in the right order, without any need for planning, control, or direct interaction between the agents. The notion of stigmergy allowed Grassé to solve the " coordination paradox " (Theraulaz & Bonabeau, 1999), i.e. the question of how insects of very limited intelligence, without apparent communication, manage to collaboratively tackle complex projects, such as building a nest.