This book attacks a major challenge in complexity theory: How do properties on the macroscopic level depend on microscopic interactions? Here, agent models have become a promising tool for simulating complex systems. In this book Brownian agents are tested on applications relating to physicochemical and biological systems, as well as to social sciences. In particular, the author investigates the influence of the internal energy depot on active particles. The book should be of interest to physicists and biologists as well as to those interested in social, urban or economic problems.