In the evolution of communication networks, there has always been a need to increase the capacity to cope with the continuous growing demand for data transmission. However, with the arrival of the Internet-of-Things and the commoditization of broadband access through smartphones, tablets, smart-watches, and all types of connecting devices, future networks must be capable of providing higher bandwidth and Quality of Experience, as wellas operating in dense networks with a massive number of simultaneous connections. This high number of connections will be very heterogeneous, spanning from highly-demanding data rate applications to low-complexity and high energy-efficient Machine-to-Machine communications. In such a dense and complex scenario, a more flexible use of spectrum resources is deemed to be the way to meet the growing requirements for data transmission. In particular, this article focuses on Device-to-Device communications and small cell deployments as emerging facilitators of such a demanding and heterogeneous scenario. The pros and cons of both complementary strategies are identified from both a technical and a business point of view, and main standardization activities are discussed. The aim of this article is to identify and describe open challenges and to inspire new areas for research that make viable the next generation of dense networks.