The molten carbonate fuel cell (MCFC) is sometimes referred to as a "mature" technology. Thereby is implied that further development of the MCFC may not be expected to yield further innovation in technology and thereby diversity in applications. However, for a clear view of the role of the MCFC in tomorrow's energy systems it makes more sense to distinguish between "scientific maturity" and "technical maturity". These concepts are discussed with special reference to the commercial prospects of the MCFC, as well as fuel cells in general. Of special interest are two cases where relative "maturity" or "immaturity" have a clear impact on potential applications, thereby either retarding or accelerating commercial development. The first case is that of the carbonate fuel cell fed by carbon, that is, a solid fuel, or by a semi-solid sludge, such as a biocarbon. The optimal design of such fuel cells is hindered by a "scientific immaturity", specifically, the lack of insight in the intrinsic kinetics of these "substrates" in molten carbonate. An illustration is provided by the difficulty of interpreting open-circuit potential measurements of carbon in molten carbonate. The second case is the one-sided emphasis on scale-up of fuel cells, whereas scale-down (miniaturization) has much to offer. A consequence of this "technical immaturity" is that, initially, there was little interest in small hybrid energy systems employing small-scale fuel cells as well as other components such as batteries, PV cells, and (super) capacitors. This lag is now finally being overcome. Some conceptual and practical examples of fuel cell/battery hybrid systems with MCFC or SOFC are given. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.