Hot cracking and hydrogen-assisted cracking are two of the most serious weld discontinuities, yet they occur for entirely different metallurgical reasons. This article explains how each type of crack forms, why hydrogen cracking can appear hours or days after welding, and how factors such as carbon equivalent, restraint, preheat, heat input, and filler metal selection influence cracking susceptibility. It also discusses practical methods for preventing both types of cracks in structural fabrication. Understanding these mechanisms helps welders, inspectors, and engineers improve weld quality while reducing costly repairs and in-service failures.