This contribution distinguishes between two conceptually different windup phenomenons, namely controller and plant windup. Even in the absence of dynamic controller elements, saturating nonlinearities can trigger undesired oscillations. This effect is obviously due to inappropriate plant states, and is therefore called plant windup. Based on the frequency response of the linear part, a design aid for plant windup prevention is presented. It facilitates the search for suitable closed-loop dynamics, or for an appropriate additional dynamic network. State and disturbance observers add dynamic elements to the compensator, and in the presence of very slow or unstable controller modes, the well-known reset windup or integral windup occurs. This effect, the controller windup, is related to mismatched controller states. By feeding the limited plant input signal into the observer it is completely removed, so that the remaining effects of input saturation are solely attributable to plant windup. Various known windup prevention techniques can be explained within the presented framework.