Abstract. In this paper, we introduce and study quotients of fully nonlinear control systems. Our definition is inspired by categorical definitions of quotients as well as recent work on abstractions of affine control systems. We show that quotients exist under mild regularity assumptions and characterize the structure of the quotient state/input space. This allows one to understand how states and inputs of the quotient system are related to states and inputs of the original system. We also introduce a notion of projectability which turns out to be equivalent to controlled invariance. This allows one to regard previous work on symmetries, partial symmetries, and controlled invariance as leading to special types of quotients. We also show the existence of quotients that are not induced by symmetries or controlled invariance. Such decompositions have a potential use in a theory of hierarchical control based on quotients.Key words. quotient control systems, control systems category, controlled invariance, symmetries AMS subject classifications. 93A10, 93A30, 93B11, 93C10 DOI. 10.1137/S03630129013990271. Introduction. The analysis and synthesis problems for nonlinear control systems are often very difficult due to the size and complicated nature of the equations describing the processes to be controlled. It is therefore desirable to have a methodology that decomposes control systems into smaller subsystems while preserving the properties relevant for analysis or synthesis. From a theoretical point of view, the problem of decomposing control systems is also extremely interesting since it reveals system structure that must be understood and exploited.In this paper we will focus on the study of quotient control systems since they can be seen as lower dimensional models that may still carry enough information about the original system. We will build on several accumulated results of different authors that in one way or another have made contributions to this problem. One of the first approaches was given in [17] were the analysis of the Lie algebra of a control system lead to a decomposition into smaller systems. At the same time in [35], quotients of control systems induced by observability equivalence relations where introduced in the more general context of realization theory. In [31], Lie algebraic conditions are formulated for the parallel and cascade decomposition of nonlinear control systems, while the feedback version of the same problem was addressed in [24]. A different approach was based on reduction of mechanical systems by symmetries. In [39], symmetries were introduced for mechanical control systems and further developed in [9] for general control systems. The existence of such symmetries was then used to decompose control systems as the interconnection of lower dimensionality subsystems. The notion of symmetry was further generalized in [26], where it was shown that the existence of symmetries implies that a certain distribution associated with the