This Master of Science Thesis deals with the optimization of well start-up procedures. Coupling of a reservoir model with a well bore flow model is the best way to capture all the complex transient flow phenomena that are predominant during such an operation. An important aspect when reading this thesis is to know that it is based on the Master of Science Thesis Optimization of well start-up by Michael Schietz. It is highly recommended to be familiar with the basic thesis to fully understand the changes and advancements described in this thesis. At the beginning an overview is given why optimization of well start-up is necessary and which problems can be avoided by doing such. The employed software and special tools offered by the used software to deal with all the phenomena encountered during start-up is described in detail. To achieve an accurate simulation of well start-up a reservoir model, a well bore flow model and an integrated model to combine those two models is necessary. The used software is SENSOR 6k (reservoir simulator), OLGA 6 GUI (well bore simulator) and Pipe-It (software especially designed for integrating and optimizing multi-component processes). The used reservoir model as well as the used well bore flow model is described in detail. Moreover the changes to achieve better results than the basic model are pointed out. The core of this thesis is the advancement to the basic model. Advancements refer to a more accurate simulation of shut in conditions and to a more complex model to simulate well start-up. The start-up simulation is now able to deal with counter current flow within the reservoir and well bore model as well as in the integrated model. Another achievement is that the model is now able to handle water in the system. It is shown that the advanced model results in better and more consistent simulations. To ensure that the new model is more advanced comparisons to the basic model are shown. This project has been carried out in close collaboration with the company PERA AS and Curtis H Whitson as supervisor. The used software was kindly provided for free by Petrostreamz AS, SPT Group Norway AS and Coats Engineering Inc. All used sources where stated using the ISO 690 convention for citation, in order to give credit to the authors and make sure that continuative research can start from this papers final state of knowledge.