Single and two phase microchannel cooling is studied by models and experiments. For single phase cooling three models are discussed: a molecular dynamics model, a Monte Carlo method and a continuum model (CFD), Also microchannel cooling experiments with water and air as coolant are discussed. The experiments and the simulations show good agreement. A pulsating heat pipe is considered as an example of two phase cooling. A one--dimensional model and experiments are compared. The trends are predicted well by the model. However, there is a large difference in absolute values between the model and the experiments. Therefore the model needs some modifications for the three-dimensional phenomena that occur in the experiments.