This article proposes a new taxonomy for describing and defining adherence to medications, aiming to address the lack of uniformity in terminology used in the field. The taxonomy is based on behavioral and pharmacological science and supports quantifiable parameters. A systematic literature review was conducted using databases such as MEDLINE, EMBASE, CINAHL, the Cochrane Library, and PsycINFO from inception to April 1, 2009, to identify different conceptual approaches to adherence research. The review identified over 10 different terms describing medication-taking behavior, often with varying meanings. The proposed taxonomy consists of three elements: 'Adherence to Medications,' 'Management of Adherence,' and 'Adherence-Related Sciences.' 'Adherence to Medications' is divided into three phases: 'Initiation,' 'Implementation,' and 'Discontinuation.' The taxonomy aims to promote consistency and quantification in terminology and methods, aiding in the conduct, analysis, and interpretation of scientific studies on medication adherence. The research was part of the ABC (Ascertaining Barriers to Compliance) project, an international collaboration funded by the European Commission.This article proposes a new taxonomy for describing and defining adherence to medications, aiming to address the lack of uniformity in terminology used in the field. The taxonomy is based on behavioral and pharmacological science and supports quantifiable parameters. A systematic literature review was conducted using databases such as MEDLINE, EMBASE, CINAHL, the Cochrane Library, and PsycINFO from inception to April 1, 2009, to identify different conceptual approaches to adherence research. The review identified over 10 different terms describing medication-taking behavior, often with varying meanings. The proposed taxonomy consists of three elements: 'Adherence to Medications,' 'Management of Adherence,' and 'Adherence-Related Sciences.' 'Adherence to Medications' is divided into three phases: 'Initiation,' 'Implementation,' and 'Discontinuation.' The taxonomy aims to promote consistency and quantification in terminology and methods, aiding in the conduct, analysis, and interpretation of scientific studies on medication adherence. The research was part of the ABC (Ascertaining Barriers to Compliance) project, an international collaboration funded by the European Commission.