Neutrinos are very weakly interacting particles with a low mass. In fact, the mass of the neutrinos has not yet been determined by experiments or observations. Still, neutrinos are a good candidate for some of the dark matter in the universe. In this thesis we will look at the effect of massive neutrinos on cosmological structure formation. We will use a model with cold dark matter (CDM) and a cosmological constant, and we will replace some of the CDM with neutrinos in the simulations. An N-body code is used to simulate the evolution of the model, and neutrinos are added to the code via two different methods. In the first method we use transfer functions from linear theory to generate a density field for the neutrinos. For the second method we will use a smoothing function that smooths out the perturbations to the CDM density field. This smoothed out field will approximate a neutrino density field, and will be added to the CDM field. We analyse the effect of the neutrinos on the matter power spectrum.