Functional near-infrared spectroscopy (fNIRS) is a promising new tool for research in human-robot interaction (HRI). The technology has already been used for brain-robot interfaces to affect robots' behaviors and as an evaluation tool for assessing brain activity during interactions. In this survey, we provide a comprehensive literature review of published research on fNIRS from various communities to assess its utility in HRI. We discuss four exemplary applications in more detail and also list several challenges that need to be overcome for fNIRS to be an effective tool in realistic HRI settings. Keywords: human-robot interaction (HRI), functional near-infrared spectroscopy (fNIRS), nearinfrared spectroscopy (NIRS), brain imaging, neuroimaging, brain-robot interface (BRI), humanmachine interaction (HMI), brain-computer interface (BCI), brain-machine interface (BMI) Authors retain copyright and grant the Journal of Human-Robot Interaction right of first publication with the work simultaneously licensed under a Creative Commons Attribution License that allows others to share the work with an acknowledgement of the work's authorship and initial publication in this journal.