The ABC model of flower development, formulated by plant developmental geneticists, has been a cornerstone in understanding how flowers are formed. Over 30 years, this model has evolved and provided insights into the regulatory processes that generate the spatio-temporal expression patterns of floral homeotic genes, the mechanisms by which ABC genes specify organ identities, and how evolution modifies the ABC model to generate morphological diversity. The model traces the history of flower development, from ancient descriptions of flower variations to modern genetic and molecular studies. Early observations of flower transformations, such as those in roses and apples, revealed homeotic changes, though the term "homeotic" was not coined until later. The connection between these observations and heritability was not always made, but with the rediscovery of Mendel, it became clear that certain flower phenotypes were heritable. The ABC model proposes three gene activities (A, B, and C) that specify the four types of floral organs. A and C activities are mutually antagonistic, and the model predicts phenotypes for double and triple mutants. The model was further supported by molecular studies, including the cloning of ABC genes and their expression patterns. The discovery of MADS-box genes and their role in floral organ identity has expanded the ABC model to include additional classes of genes, such as E-class, which are involved in specifying floral organ identity. The model has also been refined to account for variations in gene function across different plant species, with some genes showing subfunctionalization or loss. The ABC model has been further extended to include the roles of transcription factors and regulatory mechanisms that control gene expression. The model has also been shown to be conserved across angiosperms, with variations in gene function and expression patterns. The model continues to evolve, with new discoveries revealing the complexity of floral development and the molecular mechanisms underlying organ identity specification. The ABC model remains a vital framework for understanding flower development and its evolutionary origins.The ABC model of flower development, formulated by plant developmental geneticists, has been a cornerstone in understanding how flowers are formed. Over 30 years, this model has evolved and provided insights into the regulatory processes that generate the spatio-temporal expression patterns of floral homeotic genes, the mechanisms by which ABC genes specify organ identities, and how evolution modifies the ABC model to generate morphological diversity. The model traces the history of flower development, from ancient descriptions of flower variations to modern genetic and molecular studies. Early observations of flower transformations, such as those in roses and apples, revealed homeotic changes, though the term "homeotic" was not coined until later. The connection between these observations and heritability was not always made, but with the rediscovery of Mendel, it became clear that certain flower phenotypes were heritable. The ABC model proposes three gene activities (A, B, and C) that specify the four types of floral organs. A and C activities are mutually antagonistic, and the model predicts phenotypes for double and triple mutants. The model was further supported by molecular studies, including the cloning of ABC genes and their expression patterns. The discovery of MADS-box genes and their role in floral organ identity has expanded the ABC model to include additional classes of genes, such as E-class, which are involved in specifying floral organ identity. The model has also been refined to account for variations in gene function across different plant species, with some genes showing subfunctionalization or loss. The ABC model has been further extended to include the roles of transcription factors and regulatory mechanisms that control gene expression. The model has also been shown to be conserved across angiosperms, with variations in gene function and expression patterns. The model continues to evolve, with new discoveries revealing the complexity of floral development and the molecular mechanisms underlying organ identity specification. The ABC model remains a vital framework for understanding flower development and its evolutionary origins.