The development of a complex organism from a single cell is one of the most fascinating aspects in biology. We developed molecularly realistic models for basic steps in this process. Together with Alfred Gierer I have proposed that pattern formation in initially homogeneous tissues is based on reactions that combine local self-activation and long-range inhibition. Other models describe primary axes formation, pattern formation and regeneration in hydra, gene activation under morphogen control, initiation of legs and wings at the borders of differently determined cells, segmentation and somite formation due to mutual activation of locally exclusive states and the orientation of target-searching cells by week signals. The criterion for all models was that the hypothetical reactions display the same dynamics in computer simulations as found in the experiments. With the advent of the molecular-genetic approach it has turned out that the theoretically predicted reactions correspond closely to what is realized.