In this lecture on Diffusion Bonding, Roll Bonding, and Resistance Welding, we discuss three joining methods. Diffusion bonding relies on clean, smooth surfaces, pressure, temperature, time, and vacuum to promote atomic diffusion across the interface, sometimes using interlayers for better contact. Roll bonding joins stacked sheets through thickness reduction, cleanliness, and controlled rolling speed/temperature. Resistance welding uses electrical resistance heating under pressure, with current, time, and contact resistance controlling joint quality