This paper presents some needs and gaps in the representation of technology diffusion in energy-system models. Without intending to be exhaustive, some selected aspects are identified and discussed as follows: the spatial patterns of diffusion and spatial technology spillovers, the role of technology learning and R&D activities, technology clusters and uncertainty. An improved dialogue and exchange between the technology diffusion and energy-systems modeling communities is suggested, such that the representation of technology diffusion in energy-systems models is improved, thus enhancing the policy insights derived from these models.