Raman spectroscopy is a powerful tool for studying the properties of graphene. It is used to determine the number and orientation of layers, the quality and types of edges, and the effects of perturbations such as electric and magnetic fields, strain, doping, disorder, and functional groups. This provides insight into all sp²-bonded carbon allotropes, as graphene is their fundamental building block. The review discusses the state of the art, future directions, and open questions in Raman spectroscopy of graphene. It describes essential physical processes, such as various types of resonance and the role of quantum interference. The review updates all basic concepts and proposes a terminology that can describe any result in literature. It highlights the potential of Raman spectroscopy for layered materials other than graphene. Graphene is the two-dimensional building block for sp² carbon allotropes of every other dimensionality. It can be stacked into 3D graphite, rolled into 1D nanotubes, or wrapped into 0D fullerenes. It is at the center of an ever-expanding research area. Near-ballistic transport and high mobility make it an ideal material for nanoelectronics, especially for high-frequency applications. Furthermore, its optical and mechanical properties are ideal for micro and nanomechanical systems, thin-film transistors, transparent and conductive composites, and electrodes, flexible and printable (opto)electronics, and photonics. Raman spectroscopy is an ideal characterization tool, fulfilling all requirements of being fast, non-destructive, high-resolution, and applicable to both laboratory and mass-production scales. The Raman spectrum of graphite was first recorded over 40 years ago, and by the time the Raman spectrum of graphene was measured in 2006, Raman spectroscopy had become one of the most popular techniques for the characterization of disordered and amorphous carbons, fullerenes, nanotubes, diamonds, carbon chains, and poly-conjugated molecules. Raman techniques are particularly useful for graphene since the absence of a band gap makes all incident wavelengths resonant, so that the Raman spectrum contains information about both atomic structure and electronic properties. Resonance can also be reached by UV excitation, either with the M-point Van Hove singularity, or in the case of band gap opening, such as in fluorinated-graphene. The number of graphene layers in a sample can be determined by elastic light scattering/Rayleigh spectroscopy, but this approach only works for exfoliated samples on optimized substrates and does not provide other structural or electronic information. Raman spectroscopy, on the other hand, works for all graphene samples. Moreover, it is able to identify unwanted by-products, structural damage, functional groups, and chemical modifications introduced during the preparation, processing, or placement of graphene. As a result, a Raman spectrum is invaluable for quality control, and for comparing samples usedRaman spectroscopy is a powerful tool for studying the properties of graphene. It is used to determine the number and orientation of layers, the quality and types of edges, and the effects of perturbations such as electric and magnetic fields, strain, doping, disorder, and functional groups. This provides insight into all sp²-bonded carbon allotropes, as graphene is their fundamental building block. The review discusses the state of the art, future directions, and open questions in Raman spectroscopy of graphene. It describes essential physical processes, such as various types of resonance and the role of quantum interference. The review updates all basic concepts and proposes a terminology that can describe any result in literature. It highlights the potential of Raman spectroscopy for layered materials other than graphene. Graphene is the two-dimensional building block for sp² carbon allotropes of every other dimensionality. It can be stacked into 3D graphite, rolled into 1D nanotubes, or wrapped into 0D fullerenes. It is at the center of an ever-expanding research area. Near-ballistic transport and high mobility make it an ideal material for nanoelectronics, especially for high-frequency applications. Furthermore, its optical and mechanical properties are ideal for micro and nanomechanical systems, thin-film transistors, transparent and conductive composites, and electrodes, flexible and printable (opto)electronics, and photonics. Raman spectroscopy is an ideal characterization tool, fulfilling all requirements of being fast, non-destructive, high-resolution, and applicable to both laboratory and mass-production scales. The Raman spectrum of graphite was first recorded over 40 years ago, and by the time the Raman spectrum of graphene was measured in 2006, Raman spectroscopy had become one of the most popular techniques for the characterization of disordered and amorphous carbons, fullerenes, nanotubes, diamonds, carbon chains, and poly-conjugated molecules. Raman techniques are particularly useful for graphene since the absence of a band gap makes all incident wavelengths resonant, so that the Raman spectrum contains information about both atomic structure and electronic properties. Resonance can also be reached by UV excitation, either with the M-point Van Hove singularity, or in the case of band gap opening, such as in fluorinated-graphene. The number of graphene layers in a sample can be determined by elastic light scattering/Rayleigh spectroscopy, but this approach only works for exfoliated samples on optimized substrates and does not provide other structural or electronic information. Raman spectroscopy, on the other hand, works for all graphene samples. Moreover, it is able to identify unwanted by-products, structural damage, functional groups, and chemical modifications introduced during the preparation, processing, or placement of graphene. As a result, a Raman spectrum is invaluable for quality control, and for comparing samples used