This book explores the nature, classification, and use of synonyms within English phraseology, a linguistic field that studies fixed expressions, idioms, and multi‑word units. It focuses on how synonymous phraseological units function in the language — how different idioms and expressions with similar meanings can be used in various contexts to convey nuance, style, and emphasis. The book examines theoretical foundations of phraseological synonymy as well as its practical applications in translation and language teaching.