Ammonia borane has received tremendous research attention in the past decade because of its potential for chemical hydrogen storage. This paper reviews recent studies about the behavior of ammonia borane at high pressures. While much work is still needed to comprehensively understand the pressure influence on this molecular crystal, a phase diagram based on the available experimental and theoretical data is constructed. Raman spectroscopy studies indicate five transitions upon compression up to 65 GPa at ambient temperature. Diffraction experiments and theoretical studies demonstrate that three of these transitions are first-order phase transformations in the sequence of I4mm-Cmc21-P21-(different) P21, and two are iso-structural. A low-temperature phase(Pmn21) and a high-temperature high-pressure phase(Pmna) are also recognized. Ammonia borane has received tremendous research attention in the past decade because of its potential for chemical hydrogen storage. This paper reviews recent studies about the behavior of ammonia borane at high pressures. While much work is still needed to comprehensively understand the pressure influence on this molecular crystal, a phase diagram based on the available experimental and theoretical data is constructed. Raman spectroscopy tests on five transitions upon compression up to 65 GPa at ambient temperature. Diffraction experiments and theoretical studies demonstrate that that three of these transitions are first-order phase transformations in The sequence of I4mm-Cmc21-P21- (different) P21, and two are iso-structural. A low-temperature phase (Pmn21) and a high-temperature high-pressure phase (Pmna) are also recognized.