In all living organisms peptides are used to modulate key biological processes and are part of the innate immune system to fight cancer and infections. In the era of antimicrobial resistance new antibiotics are urgently needed while peptides are being rediscovered as drugs. Even though nowadays more and more peptides are in clinical trials, a detailed description of their mechanism of action is required to optimise their activity. Their flexibility and small dimensions prevent the study by powerful structural biology techniques such as X-ray diffraction and cryo-microscopy. However, solid and liquid-state NMR and Molecular Dynamic simulations can reveal their three-dimensional structure and dynamics at atomic level, even when interacting with biological membranes of entire living cells.