Chapter 10 discusses functional near-infrared spectroscopy (fNIRS), a noninvasive brain imaging technique that utilizes light to measure hemodynamic responses. It traces the evolution of spectroscopy from Newton’s prism experiments to modern neuroimaging applications, explaining how near-infrared light penetrates tissue to detect changes in oxygenated and deoxygenated hemoglobin. The chapter details the physical principles underlying fNIRS, comparing continuous wave, frequency domain, and time domain approaches while examining the instrumentation of modern systems. It addresses practical considerations including optode placement, signal quality optimization, and noise reduction techniques. The relationship between fNIRS signals and neural activity is discussed, highlighting similarities to the BOLD response in fMRI while acknowledging limitations in depth penetration. The chapter covers analytical approaches for fNIRS data processing and emphasizes its unique advantages: portability, relative affordability, and functionality in environments hostile to electromagnetic recordings. Case studies demonstrate fNIRS applications in specialized contexts like underwater environments and space exploration, illustrating why this technique has become an essential tool for specific research questions despite its spatial limitations.