Resonances of individual metabolites, such as N-acetylaspartate (NAA), can be described by parametric models. However, often a baseline, e.g. due to macromolecule resonances, makes analysis of the spectra difficult. Usually no knowledge is available about a (parametric) model for this baseline, and hence the spectra are described by a semi-parametric model. The non-parametric part of this model, namely the baseline, can be assumed to be smooth. Current work focuses on accurate model estimation and, in particular, on the determination of uncertainties. Figure 1 illustrates the non-parametric estimation of the baseline by a regularization approach.