The purpose of this report has been to design a flow controller for a Digital Displacement Pump (DDP). To achieve this result, a nonlinear model of the pump has been developed and implemented in MATLAB Simulink. The nonlinear model has been validated by using measurement data from a source at Aalborg University. At the same time, the model has been analysed to identify the sensitivity of the pump's output flow for changing oil temperature. A flow filter and flow estimator has been developed in order to estimate the output flow from the pump and to obtain its average value. Furthermore, an Internal Model Controller has been designed utilising a Smith Predictor to account for the time-delayed DDP response. The proposed controller applied on the nonlinear model showed promising results. Polynomial regression is utilised to construct an input-output map based on the nonlinear model. Additionally, two methods have been proposed to adapt the map for changing oil temperatures; an offset method and a method based on convex optimisation. Both methods utilises the predicted error to adjust the polynomial coefficients and better results were achieved by using the optimisation method on the nonlinear DDP model.