After reviewing three traditional approaches to the description of tones and their essential shortcomings in deriving the phonological structure of a tone language, the present paper describes the authors' own approach to extract both quantitative and qualitative characteristics of tones from the contours of fundamental frequency, and presents a novel framework for describing the phonological structure of the tone system of a tone language, based on the pattern of tone commands obtained from the fundamental frequency contours using the command-response model. The classification of tone command patterns is based on ternary distinction (positive-null-negative) of command polarity and binary distinction (early-late) of timing of occurrence of tone commands. The validity of the approach is demonstrated by several example languages including Mandarin, Shanghainese, Thai, Vietnamese and Lanqi. It is also shown that a finer distinction of tone command amplitude becomes necessary for the tone system of Cantonese.