This lecture covers the concept of thermo-mechanical noise in micro and nanosystems, explaining how the mass in a mechanical system is thermally coupled to the environment through damping. It discusses the spectral density of fluctuating forces, position fluctuation, and the impact of damping coefficient on energy exchange. The lecture also explores the effects of thermal agitation on mechanical systems, the minimal detectable acceleration in accelerometers, and the resolution improvement strategies. Additionally, it delves into the calibration of AFM cantilevers using mechanical-thermal noise and the quality factor scaling in NEMS, MEMS, and macro systems.