This handbook summarizes the development of concrete pressure- resistant structures for ocean applications and presents the results in the form of design guidelines. The guidelines are based primarily on test results from laboratory and ocean investigations of model concrete structures conducted at NCEL over the past two decades; the guides are principally for designing cylindrical and spherical plain (unreinforced) concrete structures to resist the externally applied pressures of hydrostatic loads and thus to be safe from implosion failure. Thin-walled and thick-walled structures are considered. For predicting implosion pressure of thin-walled cylinders, buckling expressions by Donnell for moderately long cylinders and by Bresse for long cylinders are simplified by incorporating experimentally determined numerical values for the modulus of elasticity and Poisson's ratio of high strength concrete and then modified by an empirically determined plasticity reduction factor. For thin- walled spheres, a conservative buckling expression was developed. The design approach for predicting implosion pressures of thick-walled cylinders and spheres is based on material failure in the wall of the structure; the predicted failure stress in the structure is related to the standard concrete compressive strength, f(c), by empirically derived strength increase factors. Keywords: Pressure-resistant structures, concrete spheres, concrete cylinders, submerged concrete structures, undersea structures, long-term loading, unreinforced concrete, compressive strength, cyclic loading, implosion, failure mode, durability, structural design.