Abstract. We present an extension of Eilenberg's variety theorem, a well-known result connecting algebra to formal languages. We prove that there is a bijective correspondence between formations of finite monoids and certain classes of languages, the formations of languages. Our result permits to treat classes of finite monoids which are not necessarily closed under taking submonoids, contrary to the original theory. We also prove a similar result for ordered monoids. This paper is the first step of a programme aiming at exploring the connections between the formations of finite groups and regular languages. The starting point is Eilenberg's variety theorem [10], a celebrated result of the 1970's which underscores the importance of varieties of finite monoids (also called pseudovarieties) in the study of formal languages. Since varieties of finite groups are special cases of varieties of finite monoids, varieties seems to be a natural structure to study languages recognized by finite groups. However, in finite group theory, varieties are challenged by another notion. Although varieties are incontestably a central notion, many results are better formulated in the setting of formations . This raised the question whether Eilenberg's variety theorem could be extended to a "formation theorem".The aim of this paper is to give a positive answer to this question. To our surprise, the resulting theorem holds not only for group formations but also for fo rmations of finite monoids. We also prove a similar result for formations of ordered finite monoids, extending in this way a theorem of [17] .Before stating these results more precisely, let us say a word on the aforementioned research programme and give a brief overview of the existing literature.