It is argued, on the basis of linear response theory (LRT), that new types of stochastic resonance (SR) are to be anticipated in diverse systems, quite different from the one most commonly studied to date, which has a static double-well potential and is driven by a net force equal to the sum of periodic and stochastic terms. On this basis, three new nonconventional forms of SR are predicted, sought, found and investigated both theoretically and by analogue electronic experiment: (a) in monostable systems; (b) in bistable systems with periodically modulated noise; (c) in a system with coexisting periodic attractors. In each case, it is shown that LRT can provide a good quantitative description of the experimental results for sufficiently weak driving fields. It is concluded that SR is a much more general phenomenon than has hitherto been appreciated.KEY WORDS: Analogue simulation; fluctuation phenomena; resonance; noise; spectral density; linear response; periodic attractors. 1 1. INTRODUCTION The remarkable diversity (1,2) of the systems in which stochastic resonance (SR) has already been found, or is being sought -ice-ages, lasers, electronic circuits, electron spin resonance (ESR), superconducting quantum interference devices (SQUIDs), sensory neurons, and passive optical systems, for example -is in a sense slightly misleading because, at a fundamental level, the underlying phenomenon in all of these apparently disparate cases is exactly the same. It arises because of the noise-induced increase in the system's generalised susceptibility χ(Ω) at some frequency Ω on the wing of the zero-frequency spectral peak corresponding to hopping between two (or more) static attractors (3,4) . For convenience, we shall refer to the noise-induced enhancement of a weak periodic signal in systems of this kind, where the net applied force is a sum of regular and stochastic terms, as conventional SR. The overwhelming majority of earlier work on SR (1,2) has related to conventional SR. We note that the description of conventional SR in terms of a susceptibility (3,4) , i.e. within the scope of linear response theory, has not only proven to be correct (5) , but is also simple and revealing.The aim of the present paper is to return to the interesting question of whether there may be other, quite different, classes of systems also able to support SR phenomena: that is, to explore the possibility of non-conventional SR. We shall use the latter term to describe SR in systems that do not have static potentials of the usual bistable (or multistable) type, or for which the periodic and stochastic forces are not mutually additive: in other words, we describe as non-conventional those systems which cannot be mapped into conventional SR systems by a suitable change of variable.In Section 2, we consider SR phenomena in thermal equilibrium systems with static attractors, and ask whether there may be new forms of SR not related to the zerofrequency spectral peaks associated with fluctuational transitions between the stable states of bi...