This paper reviews recent efforts in the literature to miniaturize nuclear battery systems. The potential of a nuclear battery for longer shelf-life and higher energy density when compared with other modes of energy storage make them an attractive alternative to investigate. The performance of nuclear batteries is a function of the radioisotope(s), radiation transport properties and energy conversion transducers. The energy conversion mechanisms vary significantly between different nuclear battery types, where the radioisotope thermoelectric generator, or RTG, is typically considered a performance standard for all nuclear battery types. The energy conversion efficiency of non-thermal-type nuclear batteries requires that the two governing scale lengths of the system, the range of ionizing radiation and the size of the transducer, be well-matched. Natural mismatches between these two properties have been the limiting factor in the energy conversion efficiency of small-scale nuclear batteries. Power density is also a critical performance factor and is determined by the interface of the radioisotope to the transducer. Solid radioisotopes are typically coated on the transducer, forcing the cell power density to scale with the surface area (limiting power density). Methods which embed isotopes within the transducer allow the power density to scale with cell volume (maximizing power density). Other issues that are examined include the limitations of shelf-life due to radiation damage in the transducers and the supply of radioisotopes to sustain a commercial enterprise. This review of recent theoretical and experimental literature indicates that the physics of nuclear batteries do not currently support the objectives of miniaturization, high efficiency and high power density. Instead, the physics imply that nuclear batteries will be of moderate size and limited power density. The supply of radioisotopes is limited and cannot support large scale commercialization. Niche applications for nuclear batteries exist, and advances in materials science may enable the development of high-efficiency solid-state nuclear batteries in the near term. © 2014 Elsevier Ltd. All rights reserved.