Presented have been the methods of modelling of the selected materials and operational imperfections (defects) like rotor cracks and hydrodynamic instability in rotating machines. Despite abundant information on this topic, there are still numerous problems to be solved. This regards primarily to the analysis of characteristic symptoms of such defects in the form of e.g. nonlinear vibrations and coupled forms of lateral, axial and torsional vibrations. The paper presents also author’s opinions concerning the one of the most intriguing operational imperfections, namely the formation of oil whirls and oil whips in slide bearings of a rotating machine. Coupled forms of vibration are generally a result of different kinds of couplings taking place in the system and interactions between construction and material imperfections.