This document discusses surface plasmon resonance (SPR), which is an optical technique used to study biomolecular interactions in real-time without labeling. It involves immobilizing a ligand on a gold sensor chip and passing analyte molecules over the surface. Changes in the refractive index from binding cause a change in the resonance angle, measured as response units. The sensorgram plots response over time and can reveal kinetic and affinity data. SPR is widely applied in areas like drug discovery, diagnostics, and basic research for its sensitivity, small sample size, and ability to study complex samples in real-time. While it provides valuable insights, ligand configuration may change upon immobilization and sensitivity can be limited.