Double-graphene-layer (DGL) heterostructures have recently attracted much attention due to their potential applications in high speed modulators of terahertz (THz) and infrared (IR) radiation, transistors, and THz photomixers . In this work we report experimental observation of THz emission and detection in the DGL device structures. We demonstrate that the photon-assisted resonant radiative inter-GL transitions enable the applications of such devices for THz/IR lasers and photodetectors (PDs) [2-5]. The schematic of the fabricated devices is shown in Fig. 1. The main element of both the lasers and PDs under consideration is a D-GL heterostructure with the independently contacted GLs (in red) separated by the thin transparent tunnel-barrier layer. The bias voltage V applied between the GL's contacts induces the electron and hole gases in the opposing GLs. The electron and hole densities in GLs are also controlled by the gate voltage Vg. Figure 2 show SEM image of the fabricated devices where the metal contact connected to the upper and lower graphene sheets can be seen as well as the active area of devices (700 nm × 1500 nm). The voltage-dependent band-offset energy (designated with 'Δ' in Fig. 3) between the Dirac points of the GLs and the depolarization shift determine the energies of the photons emitted (in the lasers) or absorbed (in the PDs) in the resonant-tunneling inter-GL transitions. The tunneling causes all excess charges in the n-type GL to recombine with the holes in the p-type GL. Figure 3 indicates the inter-GL radiative C-C and V-V transitions in D-GL laser (Fig. 3(a)) PD (Fig. 3(b)) structures.