In this talk, we will introduce an efficient threshold dynamics method for topology optimization for fluids modeled with the Stokes equation. The total energy decaying property of the iterative algorithm can be obtained. The extensions of the iterative thresholding method will be introduced for topology optimization for the Navier-Stokes flow, the heat transfer problems, and the heat sink problem. Then we will also introduce a recent work on the phase filed method for structural topology optimization.