The chapter introduces the field of plasmonics, focusing on the unique optical properties of noble metals. We begin by describing noble metals as plasmas, introducing concepts such as electronic screening and the ponderomotive force. We then discuss the local optical response of noble metals through their frequency-dependent dielectric function. The chapter progresses to surface plasmons at plane interfaces, detailing the properties of surface plasmon polaritons (SPPs), methods for exciting SPPs, and their practical applications in sensors, particularly their sensitivity and utility in biochemical sensing. Next, we focus on plasmons supported by nanowires and nanoparticles, utilizing the quasistatic approximation and standing plasmon waves. We analyze plasmon resonances in complex nanostructures by introducing the concept of plasmon hybridization. These resonances play a crucial role in surface-enhanced Raman scattering (SERS), enabling the detection of low-concentration analytes. We further explore nonlinear plasmonics, which leads to phenomena such as harmonic generation and four-wave mixing. Finally, we address nonlocal plasmonics, discussing the impact of spatial dispersion and mesoscopic boundary conditions on plasmonic responses at very small scales, highlighting the quantum effects at the metal surface.