Solid-state NMR is used to study insoluble proteins and proteins very sensitive to their environment such as membrane proteins and amyloid fibrils. The latter topic relates to protein aggregation diseases such as Alzheimers disease and Parkinsons disease. Solid-state NMR spectroscopy complements solution-state NMR spectroscopy and beam diffraction methods (e.g. X-ray crystallography, electron microscopy). Despite often requiring isotopic enrichment, ssNMR has the advantage that little sample preparation is required and can be used on not just dry or frozen samples, but also fully hydrated samples or native non-crystalline tissues. Solid-state NMR structure elucidation of proteins has traditionally been based on secondary chemical shifts and spatial contacts between nuclei.