Device-to-device (D2D) communication is expected to offer local area services with low transmit power and short link distance, even not via any infrastructures. These advantages will lead to the deployment of D2D systems in a massive scale, where the order of magnitude of D2D user density is higher than that of cellular user density. Network-assisted D2D systems, where D2D resources are managed by cellular networks, are unable to support the large number of D2D devices, due to the signaling overhead for control signals. In this case, no coordination can be an answer. This paper considers uncoordinated D2D systems, which is implemented with a number of D2D devices in a large scale. By analyzing the transmission capacity of D2D systems, we found a feasibility condition under which the uncoordinated D2D communications possibly coexist within cellular networks, sharing the uplink spectrum. In addition, we provide guidelines for the operational points of massive D2D communications, giving some knowledge about proper transmit power level and link distance of uncoordinated D2D.