Cold-ion populations and cold-electron populations are extremely difficult to measure in the Earth’s magnetosphere, and their properties, evolutions, and controlling factors are poorly understood. They are sometimes referred to as the “hidden populations”. But they are known to have multiple impacts on the behavior of the global magnetospheric system. These impacts include (a) the reduction of the dayside reconnection rate and consequently the reduction of solar-wind/magnetosphere coupling, (b) alteration of the growth rate and saturation amplitudes of plasma waves resulting in alterations of the energization rates of the radiation belts, (c) changes in plasma-wave properties resulting in changes in the loss rates of the ring current and radiation belts, (d) changes in the mass density of the magnetosphere resulting in changes in the radial diffusion of the radiation belts, (e) spatial and temporal structuring of the aurora, (f) altering magnetotail reconnection, (g) changing spacecraft charging, and (h) acting as sources for warm and hot magnetospheric populations. A recent workshop on the cold-particle populations of the magnetosphere inspired new work on the outstanding problems caused by a lack of understanding of those cold populations. This Research Topic will collect reports of that new work and will stimulate the formation of author teams to write review articles on what is known and what needs to be known. Commentaries assessing the present situation and guiding the research field into the future will be solicited from the community. Methods articles describing new measurement techniques and new spaceflight mission concepts will be welcomed.