This review discusses the current state of continuous wave functional near-infrared spectroscopy and imaging (fNIRS/fNIRI) instrumentation and methodology. It highlights the development of fNIRI from single-location measurements to 2D and 3D imaging, and the evolution of analysis methods from the modified Beer-Lambert law to advanced image reconstruction techniques. fNIRI has become a widely used tool in neuroscience research and is expected to become a clinical tool in the future. The review covers commercially available instruments, light sources, detectors, sensor arrangements, and methods for determining oxygenated and deoxygenated hemoglobin concentrations. It also discusses the challenges in signal separation and the importance of selecting optimal wavelengths for accurate measurements. The review emphasizes the need for high reproducibility in single subjects and the potential of fNIRI for clinical applications. The review also addresses the differences between various types of fNIRI instruments, including their light sources, detectors, and data analysis methods. The review concludes that the selection of optimal wavelengths depends on the specific analysis and assumptions made, and that future research should consider the effects of superficial layers such as skin and scalp on wavelength selection.This review discusses the current state of continuous wave functional near-infrared spectroscopy and imaging (fNIRS/fNIRI) instrumentation and methodology. It highlights the development of fNIRI from single-location measurements to 2D and 3D imaging, and the evolution of analysis methods from the modified Beer-Lambert law to advanced image reconstruction techniques. fNIRI has become a widely used tool in neuroscience research and is expected to become a clinical tool in the future. The review covers commercially available instruments, light sources, detectors, sensor arrangements, and methods for determining oxygenated and deoxygenated hemoglobin concentrations. It also discusses the challenges in signal separation and the importance of selecting optimal wavelengths for accurate measurements. The review emphasizes the need for high reproducibility in single subjects and the potential of fNIRI for clinical applications. The review also addresses the differences between various types of fNIRI instruments, including their light sources, detectors, and data analysis methods. The review concludes that the selection of optimal wavelengths depends on the specific analysis and assumptions made, and that future research should consider the effects of superficial layers such as skin and scalp on wavelength selection.