Substances can be categorized into three different states, namely, gas, liquid and solid. Among them, gas and liquid phases have the property of flowing. Since most fluids around us are transparent, we cannot see their complex behavior with our eyes directly. In the present lecture, we will introduce the flow behavior of various thermo-fluid devices, and how their shapes are optimized through prediction and understanding of flow physics. An example of optimization in biological systems, i.e., vascular network formations, and discuss the differences between the optimization procedures in engineering and biological systems will be introduced.