Abstract. In this paper we study notions of distance between behaviors of linear differential systems. We introduce four metrics on the space of all controllable behaviors which generalize existing metrics on the space of input-output systems represented by transfer matrices. Three of these are defined in terms of gaps between closed subspaces of the Hilbert space L 2 (R). In particular we generalize the "classical" gap metric. We express these metrics in terms of rational representations of behaviors. In order to do so, we establish a precise relation between rational representations of behaviors and multiplication operators on L 2 (R). We introduce a fourth behavioral metric as a generalization of the well-known ν-metric. As in the input-output framework, this definition is given in terms of rational representations. For this metric, however, we establish a representation-free, behavioral characterization as well. We make a comparison between the four metrics and compare the values they take and the topologies they induce. Finally, for all metrics we make a detailed study of necessary and sufficient conditions under which the distance between two behaviors is less than one. For this, both behavioral as well as state space conditions are derived in terms of driving variable representations of the behaviors. 1. Introduction. This paper deals with notions of distance between systems. In the context of linear systems with inputs and outputs, several concepts of distance have been studied in the past. Perhaps the most well-known distance concept is that of gap metric introduced by Zames and El-Sakkary in [28] and extensively used by Georgiou and Smith in the context of robust stability in [7]. The distance between two systems in the gap metric can be calculated, but the calculation is by no means easy and requires the solution of an H ∞ optimization problem; see [6]. A distance concept which is equally relevant in the context of robust stability is the so-called ν-gap, introduced by Vinnicombe in [22], [21]. Computation of the ν-gap between two systems is much easier than that of the ordinary gap and basically requires computation of the winding number of a certain proper rational function, followed by computation of the L ∞ -norm of a given proper rational matrix. A third distance concept is that of L 2 -gap, which is the most easy to compute but which is not at all useful in the context of robust stability, as shown in [21]. More recently an alternative notion of gap for linear input-output systems was introduced by Ball and Sasane in [13], allowing also nonzero initial conditions of the system. In this paper we will put the above four distance concepts into a more general, behavioral context, extending them to a framework in which the systems are not necessarily identified with their representations (e.g., transfer matrices), but in which, instead, their behaviors, i.e., the spaces of all possible trajectories of the systems, form