The collateral disease theory, a core theory of traditional Chinese medicine, systematically elucidates the disease patterns arising from collateral disorders. This study combines historical tracing, theoretical interpretation, and modern medical research to examine the formation and functional connotations of the qi, blood, and fluid collateral theories. It clarifies their evolution from a theoretical basis in the Huangdi Neijing to contemporary scientific validation. Physiologically, qi-collateral regulates qi and facilitates the exchange of substances. Additionally, it plays a similar role to the neuro-endocrine-immune network and alveolar functions. Blood-collateral corresponds to the distribution of microvessels, which relates to microcirculation and endothelial barrier homeostasis. Similar to lymphatic capillaries, fluid-collateral regulates body fluid metabolism. Clinically, collateral disease progression follows the sequential pattern of "initial qi involvement, chronic blood involvement, and fluid-blood mutual pathology, " with qi stagnation as the initiating factor, ultimately leading to phlegm-blood stasis and collateral obstruction and damage. Treatment refers to the theories of sugar-collateral disease, four-jiao and eight-system, and state-target differentiation, emphasizing the basic principle of unblocking collaterals. Dynamic regulation and precise intervention are performed on the three collaterals. Qi-collateral disorders require stratified treatment across the apex, upper, middle, and lower jiao. Blood-collateral disorders follow staged treatment based on the concept of "blood stagnation-blood stasis-blood concretion." Fluid-collateral disorders are addressed by regulating Xuanfu, removing stasis, and resolving dampness-phlegm. It highlights holistic spleen-stomach regulation throughout treatment, with kidney-tonifying collateral activation recommended for older and chronic patients. This study aims to provide a reference for the differentiation and treatment of collateral diseases.