One of the distinguishing features of Tai Chi is the richness of its theoretical foundation. These theories are at once the guide to improving one's practice and the measure of one's progress.
Although many of the theories seem obvious, they are not. Many seem incredibly difficult to understand, and they are.

Tai Chi practice often starts from the outside in¡ªfrom the body to the energy and mind. Later it requires going from the inside out¡ªfrom the mind and energy to the body. The process involved in this development depends largely on the individual's effort, which in turn produces insight.

Sometimes the physical effort produces insight. Sometimes there is an insight that occurs through a shift in understanding that can be applied to physical practice.
The classics are generally referred to as: The Theory of Tai Chi by Chang San Feng (Zhangsanfeng), The Classics of Tai Chi by Wong Tsung Yueh (Wangzongyue), An Internal Explanation of the Practice of the 13 Postures by Wu Yusiang (Wu Yu Xiang), The Five Words Secrets by Li Yi Yu (Li yiyu), and Summary of the Practice of Tai Chi and Push Hands by Li Yi Yu.

Additionally, there are songs, or poems, about Tai Chi, which give important suggestions about practice. Over time, highly accomplished masters have contributed their own insights. For instance, Yang Cheng-fu's 10 important points are regularly quoted. And each of the founders of major styles have left writings which have special significance.