This paper reviews optical measurements of reactive species in atmospheric-pressure nonthermal plasmas: streamer discharge, dielectric barrier discharge (DBD), plasma jet, and plasma-assisted ignition and combustion. Measurements of OH, O, N, O-3, NO, N-2(A, B, C), O-2(a, b), N-2(v), O-2(v), He*, Ar*, N-2(+), CH, and CH2O by laser-induced fluorescence, absorption, optical emission spectroscopy, and coherent anti-Stokes Raman scattering methods are included. Reactive species measurement in low-frequency (congruent to 1 Hz) pulsed streamer discharge is introduced, and reactive species production and reaction processes indicated by these measurements are described in detail. Measurements in high-frequency DBD, atmospheric-pressure diffuse discharge, and dc corona discharge are described. Measurements in plasma jets are also reviewed: rf plasma jets, kHz plasma jets, and additional plasma jets. Finally, measurements in plasma-assisted ignition and combustion are described and reviewed in addition to measurements in conventional spark ignition. A comprehensive list of the reviewed measurements is provided.