Optical on-chip data transmission enabled by silicon photonics (SiP) is widely considered a key technology to overcome the bandwidth and energy limitations of electrical interconnects. The possibility of integrating optical links into the on-chip communication fabric has opened up a fascinating new research field – Optical Networks-on-Chip (ONoCs) – which has been gaining large interest in the community. SiP devices and materials, however, are still evolving and dealing with optical data transmission on chip makes designers and researchers face a whole new set of obstacles and challenges. Designing efficient ONoCs is a challenging task and requires a detailed knowledge from on-chip traffic demands and patterns down to the physical layout and implications of integrating both electronic and photonic devices. In this paper, we provide an exhaustive review of recently proposed ONoC architectures, discuss their strengths and weaknesses, and outline active research areas. Moreover, we discuss recent research efforts in key enabling technologies, such as on-chip and adaptive laser sources, automatic synthesis tools, and ring heating techniques, whichare essential to enable a widespread commercial adoption of ONoCs in the future.