In this paper, we examine upcoming 5G networks where the support of D2D communication is expected to be a key asset for operators and users alike. First, we argue the need to functionally integrate D2D and infrastructure-to-device (I2D) modes. Next, we address practical issues such as integrated resource scheduling of D2D communication within heterogeneous networks, proposing an extension of the proportional fairness algorithm that we call multi-modal proportional fairness (MMPF). We evaluate the impact of D2D in a two-tier scenario combining macroand micro-coverage, finding that although I2D retains a clear edge for general-purpose downloading, D2D is an appealing solution for localized transfers as well as for viral content.