Animals consist of thousands of different types of molecules. To comprehend this immense complexity, we search for fundamental design principles of molecular circuits at a multi-cellular scale combining theory and quantitative experiments using C. elegans. Our focus is on the nutritional control development and aging. We ask how animals prevent that small molecular fluctuations amplify to large differences in the size during development, how organ coordinate their growth with each other, and worms they adjust their metabolism to optimize survival under nutrient starvation.