Molisch in 1937 coined Allelopathy from two Greek words, ‘allelo’ (means two organisms) and ‘pathos’ (means harm). The term defined as plant released metabolites into environment through ecological processes (volatilization, leaching, root exudation and decomposition of plant debris in soil), resulting in detrimental effects on understory or neighboring plants sharing the same habitat. This essay aimed to describe the allelopathy researches in the past, present and future, dealing with the allelopathy in natural and managed ecosystems and the allelochemicals in biomedical researches. It has 3-parts: Part I. Role of allelopathy in natural ecosystems, Part II. Allelopathy in managed ecosystems (Role in agroecology and sustainable agriculture) and Part III. Isolation and identification of allelochemicals, which exhibit bioactivities (anti-microbial, anti-hepatitis, anti-cancers and anti-diabetes to be potentially used to cure human diseases. For example, many natural products, such as flavonoids, terpenoids and alkaloids isolated from Rhododendron and Alstonia, exhibited anti-microbial activity against several microorganisms (Enterococcus faecalis, Pseudomonas aeruginosa and Klebsiella pneumoniae). The allelochemicals, ursolic acid, betulinic acid, botulin and 2,3,28-lup-20(29)-ene-triol isolated from A. scholaris, showed the proliferative activity of non-small-cell-lung cancer (NSCLC). The ursolic acid isolated from R. formosanum also showed pronounced growth inhibition of NSCLC, in addition, cinnamtannin D1 also exhibited anti-cancer efficacy that induced G1 arrest and autophagy via the mTOR pathway inhibition and ERK 1/2 pathway activation. Several Solanum species are traditionally used to treat HBV infection. In Cucurbitace family, fruits and stems of Momordica charantia (called bitter melon) possess anti-diabetic and anti-lipedemic compounds. This essay also describes the mode of actions of the aforementioned allelochemicals in the metabolomic pathway of anticancer or antimicrobial activities. The authors create a new scientific term called “Allelochemicology”, which hopefully, inspires young scientists in life sciences to explore this field, to solve the most difficult tasks on ecology, sustainable agriculture, pharmaceutical and medicine that can be beneficial to human beings for reaching the Sustainable Development Goals. © 2023, International Allelopathy Foundation. All rights reserved.