This is the first comprehensive study of phraseological borrowing. Based on Latvian material, it reaches far beyond one language. It describes the reasons and motivations, processes and results of the borrowing of phraseological units; examines general theories of phraseology, language contact and borrowing. On the basis of empirical material it sets out a system of phraseological borrowing comprising four types of phraseological loans - phraseological translation loans, full phraseological loans, phraseological semicalques and phraseological transcription loans. It also examines the development of the Latvian system of phraseology and discusses borrowings from various languages separately, including many examples. Borrowing is one of the main ways of phraseology formation. Entire layers of phraseology in most European languages are phraseological loans and internationalisms, most visible, for example, in ancient mythology and biblical phraseology. Borrowed units are adapted, change meanings and form. Native language structures indirectly affect them, as is evident in the assimilation phase, when many units adapt to the language, its expectancy norms and conventions.