In petrochemical industry, elevated flares are the important devices to burn off the waste gas from upstream processes. However, there often exist incomplete combustions during flare operations. These are mainly manifested as two forms: (1) unburned volatile organic compound (VOC) emission due to low heat values in the combustion zone; (2) smoke or soot emission due to insufficient air supply. In recent years, the flare industry has made an attempt to use optical methods on flares to monitor their combustion conditions. Several optical monitoring technologies including passive Fourier Transform Infrared (PFTIR) spectroscopy, mid-infrared hyper-spectral imagery, sky-light line-of-sight attenuation (sky-LOSA) and dual wavelength infrared sensor used in flare industry are presented. Their operating principles, applications and developments are discussed. Finally, the development trend and application prospect of remote optical monitors in the field of elevated flares are analyzed.