In recent years microcalorimeters and bolometers have achieved impressive performance in terms of energy resolution or Noise Equivalent Power (NEP). However their performance is still significantly worse than predicted by ideal models. The difference is due primarily to non-ideal effects that have to be taken into account and to additional noise of unknown origin. The most significant non-ideal effects have been quantified and include hot-electron and absorber decoupling and the related extra noise sources, and non-ohmic behavior of the thermometer. Excess noise of unknown origin is particularly significant when Transition Edge Sensors (TES) detectors are used as thermometers and is currently under experimental and theoretical investigation. This paper reviews the origin of non-ideal effects in microcalorimeters and bolometers and their effect on energy resolution and NEP. It also reviews the status of theoretical and experimental results on the characterization of the excess noise in TES, and in particular its relation to fundamental physics in superconductors near the transition temperature.