This interdisciplinary book covers the fundamentals of optical whispering gallery mode (WGM) microcavities, light–matter interaction, and biomolecular structure with a focus on applications in biosensing. Novel biosensors based on the hybridization of WGM microcavities and localized surface plasmon resonances (LSPRs) in metal nanoparticles have emerged as the most sensitive microsystem biodetection technology that boasts single molecule detection capability without the need for amplification and labeling of the analyte. The book provides an ample survey of the physical mechanisms of WGMs and LSPRs for detecting affinity, concentration, size, shape and orientation of biomarkers, while informing the reader about different classes of biomolecules, their optical properties and their importance in label-free clinical diagnostics.