While working at CERN, he realized that reggeon field theory can be viewed as a contact process in the same universality class as directed percolation. After making this discovery, Grassberger turned his attention to the studies of statistical physics, dynamical systems, sequential sampling algorithms, and complex systems. His publications span a variety of topics including reaction-diffusion systems, cellular automata, fractals, Ising model, Griffiths phases, self-organized criticality, and percolation.