This publication examines the phenomena of delayed hydride cracking (DHC) in Zircaloy-4 claddings and presents the results of a coordinated research project on this subject. It discusses the circumstances and the mechanisms that lead to DHC in Zircaloy-4 cladding materials and documents the knowledge gained through an inter-laboratory comparison where the physical conditions for DHC have been studied. An advanced DHC testing methodology developed in Studsvik, Sweden, and consistent experimental data of international round-robin DHC velocity measurements in CANDU/PHWR and LWR cladding Zircaloy-4 materials are also attached.