The author's view on tropical cyclones as the conditional instability of the second kind (CISK) is presented. Many theoretical and numerical studies of tropical cyclones have discussed the original CISK of Ooyama (1964, 1969) and Charney and Eliassen (1964) in which frictional convergence plays an important role. In this paper, the author emphasizes that this CISK is applied primarily to the eyewall circulation in intense tropical cyclones, and describes other types of CISK, which are appropriate to explain many of tropical disturbances and tropical depressions, and the formation and early development stages of tropical cyclones. In addition, the importance of resolving mesoscale organized convection in coarse-resolution numerical models is emphasized.