Firstly proposed in 1995 and systematically developed in the past decade, Bayesian YingYang learning 1) is a statistical approach for a two pathway featured intelligent system via two complementary Bayesian representations of a joint distribution on the external observation X and its inner representation R, which can be understood from a perspective of the ancient Ying-Yang philosophy. We have q(X, R) = q(X|R)q(R) as Ying that is primary, with its structure designed according to tasks of the system, and p(X, R) = p(R|X)p(X) as Yang that is secondary, with p(X) given by samples of X while the structure of p(R|X) designed from Ying according to a Ying-Yang variety preservation principle, i.e., p(R|X) is designed as a functional with q(X|R), q(R) as its arguments. We call this pair Bayesian Ying-Yang (BYY) system. A YingYang best harmony principle is proposed for learning all the unknowns in the system, in help of an implementation featured by a five action circling under the name of A5 paradigm. Interestingly, it coincides with the famous ancient WuXing theory that provides a general guide to keep the A5 circling well balanced towards a Ying-Yang best harmony. This BYY learning provides not only a general framework that accommodates typical learning approaches from a unified perspective but also a new road that leads to improved model selection criteria, Ying-Yang alternative learning with automatic model selection, as well as coordinated implementation of Ying based model selection and Yang based learning regularization.This paper aims at an introduction of BYY learning in a twofold purpose. On one hand, we introduce fundamentals of BYY learning, including system design