Roughly based on the historical order of advances in tidal theory, the classical theory that involves a 2D steady and variable separable basic partial differential equation, which is reduced from a set of starting equations linearized under an assumption without dissipations and background winds, and its global wave mode solutions expanded in Hough functions was reviewed at first. Then a modern numerical model (i.e., GSWM), which was mainly developed by Forbes and Hagan beginning from a linearization under realistic conditions of various dissipations and nonzero background winds and so is still 2D steady in essence but cannot be solved by the method of separation of the latitude and height variables, was introduced. At last, a developing tendency that people have begun in recent years to employ some GCM (e.g. TIME-GCM, MUAM, CMAM and WACCM), which pay more attentions to the middle and upper atmosphere, to study the tidal nonlinearity and nonmigrating tides etc. was summarized. Also given are some analyses about the characteristics of respective tidal models and differences between them by incorporating the understanding of the author. Along the way, it can be seen that during half a century starting from first principles people have made persistent efforts to disclose the realistic structural characteristics of the global atmospheric tides and the mechanisms for tidal long- and short-term variability, while this kind of endeavor is closely connected with the advances in observations of atmosphere especially the wind and temperature fields in the MLT by using from ground-based radars to instruments on board satellites.