Delayed hydride cracking (DHC) in zirconium alloys arises as a consequence of the diffusion of hydrogen atoms to a crack tip, precipitation of hydride platelets and then the fracture of a hydrided region that has formed ahead of the crack tip. This process repeats itself and, consequently, a crack grows in a series of steps. There is a threshold value, K-IH, of the crack tip stress intensity below which DHC crack growth is unable to proceed. The present paper provides a physical picture of the near threshold situation, accounting systematically for the manner in which hydrided material fractures, and consequently obtains an expression for K-IH in terms of the hydrided material’s flow and fracture characteristics.