—How would a cellular network designed for high energy efficiency look like? To answer this fundamental question , we model cellular networks using stochastic geometry and optimize the energy efficiency with respect to the density of base stations, the number of antennas and users per cell, the transmit power levels, and the pilot reuse. The highest efficiency is neither achieved by a pure small-cell approach, nor by a pure massive MIMO solution. Interestingly, it is the combination of these approaches that provides the highest energy efficiency; small cells contributes by reducing the propagation losses while massive MIMO enables multiplexing of users with controlled interference. I. INTRODUCTION Two key goals for the fifth generation (5G) cellular networks are improved spectral efficiency (SE) and higher energy efficiency (EE) [1]. These performance metrics are coupled and cannot be treated separately in the design of future networks [2]. The key to improve the SE and EE is higher spatial reuse; that is, more parallel transmissions per km 2. There are two main densification approaches: 1) smaller cell radius [3] and 2) massive MIMO (multiple input, multiple output) technology [4]. The purpose of this paper is to show that these approaches are fundamentally non-competing; in fact, both are needed to make future wireless networks truly energy efficient. The EE is defined as the benefit-cost ratio of the network: