The first part of the present report contains a study in which the stability of very simplified models of a Darrieus rotor is considered. The influence of various structural parameters concerning tower and blades is investigated. The second part deals with the stability calculations of a strip rotor, i.e. a rotor with straight blades, representing more or less the 5 m Darrieus rotor. Results show elastomechanical and aeroelastic instabilities well below the maximum operational speed. Finally, a calculation of the elastomechanical stability of the 5 m Darrieus rotor is discussed, based on vibration modes determined with the finite element method. The results correspond to those of the strip rotor. The aeroelastic stability calculations for the 5 m Darrieus rotor has not been finished as yet. Some recommendations for improving the stability are given.