Biophysical studies of membrane proteins face multiple obstacles including low expression yield, sample heterogeneity and the need for a membrane mimetic that resemble the native environment. When using when using NMR spectroscopy additional problems are the large size of the systems, the need of incorporating stable isotope 13C, 15N and 2H. Micelles or bicelles are frequently used for structural studies of membrane proteins but have the disadvantage of the destabilizing effect of the detergents, in particular when the membrane proteins have water-soluble domains or interact with soluble proteins. Therefore, we embraced the use of phospholipid nanodiscs for membrane protein studies. Nanodiscs are patches of phospholipid bilayer surrounded by two copies of a membrane-scaffolding protein (Msp1), which is derived from apolipoprotein A1. We had shown that the Voltage-Dependent Anion Channel (VDAC1) can be inserted into nanodiscs and can be analysed both in EM images and in NMR spectra. However, embedding was heterogeneous and prevented detailed NMR analysis. To address this problem and adapt nanodiscs to different size membrane proteins we first engineered Msp1 variants that yield nanodiscs at diameters of 9.5, 8.1, 7.8 and 6.8 nm, and we could determine an NMR structure of OmpX in the 8.1 nm nanodisc. However, the diameter distribution of these nanodiscs is rather wide. To further optimize nanodiscs we developed procedures to covalently circularize the scaffolding protein and make nanodiscs of exactly defined diameters. We could extend the cND sizes from as low as 8 nm to as high as 50 nm each exhibiting a very narrow size distribution. We are able to insert membrane protein into the covalently circularized nanodiscs, record NMR spectra for smaller systems and obtain negative stain or cryo EM images from nanodisc-bound membrane proteins. We pursued applications from small mitochondrial membrane proteins to GPCRs and larger systems.