Composites represent a class of materials with properties that are obtained by combining the functions of different components. Combining soft and stiff components without losing toughness is typically very difficult with current synthetic tools. There are many natural materials for which this problem has been solved. Examples such as wood and seashells have inspired many scientists to seek tougher, stronger and lighter materials. This review describes how genetic engineering can help in building new composites with better properties. Specifically, we emphasize that functional molecules can be engineered by following the design principles of natural composite materials. This field is emerging but has already shown promising results and much progress in the next few years is expected.