Isaeus was a professional speechwriter, or logographer, who worked in Athens during the first half of the fourth century BCE. He was a metic, a resident foreigner, and ancient sources suggest he may have come from Chalcis. His career focused exclusively on writing forensic speeches for private court cases, particularly complex disputes over inheritance and family property. According to ancient tradition, he was a pupil of the famous rhetorician Isocrates and later a teacher of Demosthenes.

None of the speeches he wrote for clients were delivered by him personally. Of the many speeches attributed to him, twelve survive essentially complete, all dealing with inheritance law. These works provide detailed examples of Athenian legal practice. Modern scholars value them as crucial historical sources for understanding family relationships, property rights, and social strategies in classical Athens. His style was noted for its clear argumentation and methodical use of evidence, including laws and witness testimony. While some ancient critics found his speeches lacking in emotional power, they praised his logical skill and pure Attic Greek. His technical expertise in constructing legal arguments is considered a significant influence on the development of Athenian oratory.