The vast amount of published work on chemical modification in electrothermal atomic absorption spectrometry (about 700 papers) has been critically examined and reviewed. Until now the chemical modification technique has been applied to the determination of 47 analyte elements in various matrices, and a large variety of different chemical modifier combinations (more than 150) have been found in the literature. An attempt has been made to organize and classify this large amount of empirical information. Various theoretical and experimental approaches have been used, such as the guiding and prediction power of the Periodic Table; the concept of isomorphism between analyte-analyte, analyte-modifier and modifier-modifier pairs; multivariate methods based on several sets of fundamental parameters and experimental data for the analyte or modifier elements: cluster, correlation, discriminant and factor analysis; thermodynamic calculations; data for the electronic configuration of analyte elements of their ions; and experimental data on maximum thermal pre-treatment temperatures in the presence of series of chemical modifiers. By using these various techniques, classifications for chemical modifiers and analyte elements have been proposed and discussed.