The aim of this paper is to study the existence of limit-cycles in a closed loop, which simultaneously contains nonlinearities and parametric uncertainties. Three methods are presented : we first consider the problem of detecting a limit-cycle using a necessary condition of oscillation. A graphical method and a first mu based method are proposed. We then consider the problem of checking the absence of limit-cycles despite parametric uncertainties. A second mu based method is proposed, which uses a sufficient condition of non-oscillation. An example is finally presented : the idea is to use the necessary condition of oscillation, so as to synthesize a controller which modifies the characteristics (magnitude and frequency) of the limit-cycle.