Electric vehicles (EVs) are currently promoted by government and industry as an alternative to traditional internal combustion engine (ICE) propelled vehicles. However, e-mobility has not yet reached a level of technological maturity that allows for the same degree of mobility offered by ICE propelled vehicles. In this paper, we argue that the adaption of roaming concepts to enable EV charging across geographical and service provider boundaries is likely to be a crucial element for e-mobility to be successful. To that end, we derive requirements and design principles for roaming concepts in e-mobility. Furthermore, we present and briefly evaluate a prototype implementation of an e-mobility roaming platform as a proof-of-concept.