In the biomedical field there is the need of providing physicians with quantitative information in a fast and reliable way for patients' diagnosis and follow-up, preferring real-time, non-invasive, non-ionizing, portable and low cost techniques, when possible. Near Infrared Spectroscopy (NIRS) presents all of these advantages and is mostly used to monitor cerebral autoregulation and muscle oxidative metabolism: its applications continue to broaden as multi-channel and portable devices have permitted to deepen the exploration of physiological phenomena and obtain better temporal and spatial resolution with long duration acquisitions; in the future it is expected that NIRS devices will have a key role in brain computer interfaces and diagnostic purposes.