This review article, published in *NeuroImage* in 2014, provides a comprehensive overview of continuous wave functional near-infrared spectroscopy (fNIRS) and imaging (fNIRI) instrumentation and methodology. The authors, Felix Scholkmann, Stefan Kleiser, Andreas Jaakko Metz, Raphael Zimmermann, Juan Mata Pavia, Ursula Wolf, and Martin Wolf, highlight the advancements in fNIRS/fNIRI technology over the past 20 years, focusing on the commercial instruments available and the underlying principles of light sources, detectors, and sensor arrangements. The article traces the historical development of fNIRS from early single-location measurements to advanced imaging techniques, including two-dimensional (topographic) and three-dimensional (tomographic) imaging. It discusses the evolution of analysis methods, from the simple modified Beer-Lambert law to sophisticated image reconstruction and data analysis techniques. Key aspects covered include: - **Light Sources**: Discrete and broadband light sources, their spectral characteristics, and safety considerations. - **Detectors**: Photodiodes, avalanche photodiodes, and photomultiplier tubes, with a focus on their performance and noise characteristics. - **Sensor Arrangements**: Techniques for optimizing light coupling and reducing artifacts, such as spring-loaded probes and fiber brushes. - **Data Analysis**: Algorithms for calculating hemoglobin concentrations and methods for separating different components in fNIR signals. The authors also discuss the challenges and future directions in fNIRS/fNIRI, emphasizing the need for improved reliability in single-subject measurements and the potential for clinical applications. The review concludes with an extensive overview of commercially available fNIRI devices, detailing their features, capabilities, and limitations.This review article, published in *NeuroImage* in 2014, provides a comprehensive overview of continuous wave functional near-infrared spectroscopy (fNIRS) and imaging (fNIRI) instrumentation and methodology. The authors, Felix Scholkmann, Stefan Kleiser, Andreas Jaakko Metz, Raphael Zimmermann, Juan Mata Pavia, Ursula Wolf, and Martin Wolf, highlight the advancements in fNIRS/fNIRI technology over the past 20 years, focusing on the commercial instruments available and the underlying principles of light sources, detectors, and sensor arrangements. The article traces the historical development of fNIRS from early single-location measurements to advanced imaging techniques, including two-dimensional (topographic) and three-dimensional (tomographic) imaging. It discusses the evolution of analysis methods, from the simple modified Beer-Lambert law to sophisticated image reconstruction and data analysis techniques. Key aspects covered include: - **Light Sources**: Discrete and broadband light sources, their spectral characteristics, and safety considerations. - **Detectors**: Photodiodes, avalanche photodiodes, and photomultiplier tubes, with a focus on their performance and noise characteristics. - **Sensor Arrangements**: Techniques for optimizing light coupling and reducing artifacts, such as spring-loaded probes and fiber brushes. - **Data Analysis**: Algorithms for calculating hemoglobin concentrations and methods for separating different components in fNIR signals. The authors also discuss the challenges and future directions in fNIRS/fNIRI, emphasizing the need for improved reliability in single-subject measurements and the potential for clinical applications. The review concludes with an extensive overview of commercially available fNIRI devices, detailing their features, capabilities, and limitations.