Radio communications have now widely penetrated both the professional and private spheres, for everyday uses of largely differing importance and criticality that are anticipated to further increase in the coming years through 5G networks and the internet of things. This will address major “vertical” applications such as smart homes and smart cities, agriculture, connected and autonomous vehicles, health, industry and many others. While in some cases the failure of the radio link is tolerable and without serious implications, in others reliability will be critical and unavoidable. Unfortunately, the laws of electromagnetics are such that numerous effects are detrimental to the link budget, are varying and are uneasily controllable. In this context, how to ensure a very high level reliability, which solutions are feasible and up to which degree of complexity? This paper recalls the physical and technical sources of an uncertain reliability for the radio link, in relation with antennas and propagation, and evokes “massive diversity” as a future development to guarantee a constant connectivity service with ultra-high reliability.