Transient luminous events (TLEs) are large scale optical events occurring at high altitudes, which are related to the electrical activity in underlying thunderstorms. The known TLEs can be classified into four categories, Red Sprites that develop at the base of the ionosphere and move rapidly downward; Blue Jets, which emanate from the top of thunderclouds up to an altitude of 40 km; ELVEs, which occurred in the lower ionosphere region and can spread over 300 km laterally; and upward moving Gigantic Jets, which establish a direct path of electrical connection between thundercloud tops and the lower ionosphere. An attempt was made to summarize the general phenomenology on morphological features of TLEs based on the available experimental data and the interpretation of these observed features with modeling efforts. The effects of TLEs on the stratosphere, mesosphere and ionosphere were discussed. Finally, a need for observation aimed data accumulation and further theoretical study of TLEs were identified, and some presently unsolved problems in the theory of TLEs were outlined.