The thermal behavoir of permanent magnet synchronous machines (PMSMs) is important since overheating of the permanent magnets can lead to demagnetization. This article gives an overview of a hybrid thermal model for a high speed PMSM which combines lumped parameter and analytical models. The losses found in a slotless PMSM are also discussed since they are the heat sources. An overview of the experiments performed to determine some of the thermal parameters and verify the models are also given. It is shown that bearing loss can make a significant contribution to the thermal behavior of a machine.