This book reports the recent study results in applying online signals and artificial intelligence for welding and laser additive manufacturing (AM). It provides insights on the fundament of quality monitoring and intelligent defect detections for welding and laser AM using arc sound and plasma spectral signals collected during the manufacturing processes. Analyzing respectively from a global and local perspective establishes a knowledge base of features characterizing welding statuses. Researchers, scientists, and engineers in intelligent welding and laser AM can benefit from the book. As such, this book provides valuable knowledge, valuable methods, and practical algorithms that can be considered in real-time defect detection for welding and laser AM.