## 1 Introduction  
Why do electron-phonon (e-ph) interactions matter? This question is undoubtedly the start-point to understand many fundamental phenomena in solids. Usually, the states of matter are principally studied in terms of electronic degrees of freedom in materials. However, the coupling of the electronic degrees of freedom to lattice degrees of freedom is crucial to understanding materials properties. This coupling is usually described by interactions between the electronic excitations and phonons, and is responsible for many interesting effects in a crystal, such as the formation of polarons, formation of Cooper pairs and the superconductivity, the Peierls transition and the Kohn anomaly, an abrupt softening of phonon energies [1]. Electron-phonon interactions can be observed in a diversity of materials. This review focuses on the study of intra- and interlayer e-ph interactions, as well as their consequences, in bilayer graphene (2 LG) systems, which have been shown as ideal platforms to observe, for example, the Kohn anomaly effect mentioned above [1]. Graphene is a special material due to its fascinating electrical, mechanical, optical and thermal properties [2; 3; 4; 5]. This strictly two-dimensional material exhibits exceptionally high crystal and electronic quality, in which charge carriers can travel thousands of interatomic distances without scattering [5; 6], and has revealed a great deal of new physics and potential technological applications. Particular interest has been turned to single layer graphene (1 LG) due to the unique nature of its charge carriers, that make it a promisor material for photonics, optoelectronics and organic electronics such as in solar cells, light-emitting, touch screen and photodetectors devices [2; 7; 8]. Here, the authors will discuss mostly 2 LG, which is also a zero gap semiconductor and it is a highly desired material for the development of graphene-based electronics such as field effect transistor [5; 6], since it becomes a tunable band gap semiconductor under the application of an electric field perpendicular to the layers composing system [9; 10]. Recently, advances in chemical vapor deposition (CVD) allowed the realization of twisted and AB-stacked 2 LG in which top and bottom layers are compose solely of either \({}^{13}\)C or \({}^{12}\)C. This opens up a completely new route to combine phonons and electrons in this layered systems. Because \({}^{13}\)C and \({}^{12}\)C are isotopes, these hybrid bilayer systems keep the same electronic behavior observed for \({}^{12/12}\)C 2 LG although the phonons can behave quite differently [11]. The 2 LG systems are bound together by means of weak interlayer (IL) interactions mediated by van-der-Waals (vdW) forces. These IL interactions are sensitive to the number of layers and stacking order and are important to technological applications of these systems [12; 13; 14; 15; 16; 17; 18]. Namely, the IL interactions govern the low-energy electronic and vibrational properties, which are of primary importance to phenomena such as transport, infrared optics and telecommunication bands in the infrared (IR) range [12; 13; 14]. In 2 LG, although the effects of the IL interactions on the electronic properties are well understood [19; 20], it was only recently that the understanding of IL-related vibrational properties, electron-electron (e-e), phonon-phonon (ph-ph), and electron-phonon (e-ph) interactions started being developed [21; 22; 23; 24; 25]. Such development has already impact this research field, by opening a route to understanding IL interactions in similar, but more complex 2D-layered materials, such as MoS\({}_{2}\), WSe\({}_{2}\), oxides and hydroxides [26; 27; 28; 29]. In this manuscript, the use of gate-modulated Raman spectroscopy to characterize 2 LG and to have information about their IL interactions is discussed. Traditionally, e-ph interactions are investigatedthrough chemical doping, in which the charge carrier density is varied by the introduction of impurities [30].