Marcus Bellander present his master thesis Non-linear optimization using Constrained Neural Networks with Applications on the Nordic Energy Market Abstract In this thesis we study time series modeling and forecasting by using feed-forward neural networks. Neural networks are data-driven universal approximators, i.e. black box-models. The scope of the thesis has been to incorporate a priori knowledge in the training of a neural network, thus creating a gray box-model. A method of analyzing such a model is presented, as well as a demonstration of these methods on both synthetic and real problems from the Nordic Energy Market.