This paper presents ultrafast photodetectors based on single and few-layer graphene. The photodetectors demonstrate a photocurrent response up to 40 GHz, with no degradation observed at the highest measured frequency. The intrinsic bandwidth of such graphene-based photodetectors is estimated to exceed 500 GHz. The unique properties of graphene, including its high carrier mobility and zero dark current operation, enable high-speed photodetection. The generation and transport of photo-carriers in graphene photodetectors differ fundamentally from those in conventional semiconductor photodetectors, leading to a high bandwidth, zero source-drain bias operation, and good internal quantum efficiency. Graphene is a single atomic layer of carbon arranged in a hexagonal lattice, with exceptional electronic and optical properties. It has a linear energy dispersion relation, high Fermi velocity, and high electrical mobility. Its photonic properties are also remarkable, with strong inter-band transitions and wide absorption range. The combination of these properties makes graphene suitable for novel photonic devices. The photodetectors are fabricated using mechanical exfoliation of graphite and are similar to standard back-gated graphene transistors. The devices are characterized using high-frequency electrical and optical measurements. The results show that the graphene photodetectors have a high-frequency AC photo-response up to 40 GHz, with no degradation observed. The internal quantum efficiency is estimated to be 6-16%, and the external quantum efficiency can be improved by increasing the photo-detection area or reducing internal resistance. The photodetectors operate without a source-drain bias, leading to zero dark current and high bandwidth. The mechanism of photo-carrier generation and collection is different from conventional photodetectors, allowing for high-speed operation. The transit time limited bandwidth of the photodetector is calculated to be 1.5 THz, which is much higher than that of conventional semiconductors. The paper also discusses the internal and external quantum efficiency of graphene photodetectors, showing that the external efficiency can be improved by increasing the photo-detection area or reducing internal resistance. The results demonstrate the potential of graphene for high-speed photonic applications, including optical communications, interconnects, terahertz detection, imaging, remote sensing, and spectroscopy.This paper presents ultrafast photodetectors based on single and few-layer graphene. The photodetectors demonstrate a photocurrent response up to 40 GHz, with no degradation observed at the highest measured frequency. The intrinsic bandwidth of such graphene-based photodetectors is estimated to exceed 500 GHz. The unique properties of graphene, including its high carrier mobility and zero dark current operation, enable high-speed photodetection. The generation and transport of photo-carriers in graphene photodetectors differ fundamentally from those in conventional semiconductor photodetectors, leading to a high bandwidth, zero source-drain bias operation, and good internal quantum efficiency. Graphene is a single atomic layer of carbon arranged in a hexagonal lattice, with exceptional electronic and optical properties. It has a linear energy dispersion relation, high Fermi velocity, and high electrical mobility. Its photonic properties are also remarkable, with strong inter-band transitions and wide absorption range. The combination of these properties makes graphene suitable for novel photonic devices. The photodetectors are fabricated using mechanical exfoliation of graphite and are similar to standard back-gated graphene transistors. The devices are characterized using high-frequency electrical and optical measurements. The results show that the graphene photodetectors have a high-frequency AC photo-response up to 40 GHz, with no degradation observed. The internal quantum efficiency is estimated to be 6-16%, and the external quantum efficiency can be improved by increasing the photo-detection area or reducing internal resistance. The photodetectors operate without a source-drain bias, leading to zero dark current and high bandwidth. The mechanism of photo-carrier generation and collection is different from conventional photodetectors, allowing for high-speed operation. The transit time limited bandwidth of the photodetector is calculated to be 1.5 THz, which is much higher than that of conventional semiconductors. The paper also discusses the internal and external quantum efficiency of graphene photodetectors, showing that the external efficiency can be improved by increasing the photo-detection area or reducing internal resistance. The results demonstrate the potential of graphene for high-speed photonic applications, including optical communications, interconnects, terahertz detection, imaging, remote sensing, and spectroscopy.