Phytochemicals, also known as plant secondary metabolites, play primary roles in plant development, structure, and response to the environment. Their metabolism has been studied for over a century. In a new review, Dixon and Dickinson discuss past and recent advances in understanding the identity, localization, biosynthesis, and functions of phytochemicals over the past century. One of the topics of this review is the importance of plant secondary metabolites in human medicine and agricultural biotechnology and their other potential benefits for society. The authors also highlight how secondary metabolites are used by plants in defense, development and chemical ecology. Another theme in the review is the techniques used over the years in the discovery and analysis of secondary metabolites, from classic chemistry to high-throughput gene sequencing, mass spectrometry, AI, and synthetic biology. Other newly emerging technologies, such as single-cell omics and small-molecule biosensors are also discussed, as are areas needing further exploration.