Attention deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder that affects children and adults worldwide causing impairment in their social, academic, or occupational functioning. Several studies have shown that the disorder is characterized by alterations in brain functioning. Thus, functional neuroimaging techniques such as functional near-infrared spectroscopy (fNIRS) can contribute to the elucidation of the disorder. fNIRS is a noninvasive neuroimaging technique that employs light in the near-infrared spectrum to assess brain activity in the cortex by quantifying the amount of light absorbed by hemoglobins. The technique has advantages and disadvantages in comparison to other neuroimaging instruments, but it is considered a reliable measure. Studies using fNIRS in ADHD usually focus on prefrontal cortex functions although some other brain regions associated with executive functions and other cognitive deficits have been investigated as well. Overall, fNIRS have helped in understanding cognitive deficits in ADHD and showed the efficacy of treatments and even was implemented as part of neurofeedback protocols. fNIRS is a valuable neuroimaging technique to understand ADHD and other neurodevelopmental disorder.