Amorphous calcium carbonate (ACC) is regularly listed as one of the known calcium carbonate phases, first being identified in 1984. Although ACC is inherently unstable, it is a key material in biomineralisation of crystalline calcium carbonate, for example in mollusc and crustacean shells. In the past 25 years there have been dozens of studies of ACC, including "synthetic" ACC prepared in the laboratory. Techniques such FTIR and Raman spectroscopy, and X-ray diffraction have frequently been used but no neutron diffraction has been reported. A reverse Monte Carlo model for ACC based on X-ray diffraction data has just been reported in 2010. It is very timely to collect neutron diffraction data which will complement the X-ray diffraction data, and will be essential for establishing a firm understanding of the structure of ACC.