We introduce a criterion for the existence of regular states in systems with a mixed phase space. If this condition is not fulfilled chaotic eigenstates substantially extend into a regular island. Wave packets started in the chaotic sea progressively flood the island. The extent of flooding by eigenstates and wave packets increases logarithmically with the size of the chaotic sea and the time, respectively. This new effect is observed for the example of island chains with just 10 islands.PACS numbers: 05.45. Mt, 03.65.Sq One of the cornerstones in the understanding of the structure of eigenstates in quantum systems is the semiclassical eigenfunction hypothesis [1]: in the semiclassical limit the eigenstates concentrate on those regions in phase space which a typical orbit explores in the longtime limit. For integrable systems these are the invariant tori. For ergodic dynamics the eigenstates become equidistributed on the energy shell [2]. Typical systems have a mixed phase space, where regular islands and chaotic regions coexist. In this case the semiclassical eigenfunction hypothesis implies that the eigenstates can be classified as being either regular or chaotic according to the phase-space region on which they concentrate. Note, that this may fail for an infinite phase space [3].In this paper we study mixed systems with a compact phase space, but away from the semiclassical limit. Here the properties of eigenstates depend on the size of phasespace structures compared to Planck's constant h. In the case of 2D maps this can be very simply stated [4]: a regular state with quantum number m = 0, 1, ... will concentrate on a torus enclosing an area (m + 1/2)h, as can be seen in Fig. 1(c).We will show that this WKB-type quantization rule is not a sufficient condition. We find a second criterion for the existence of a regular state on the m-th quantized torus,Here τ H = h/∆ ch is the Heisenberg time of the chaotic sea with mean level spacing ∆ ch and γ m is the decay rate of the regular state m if the chaotic sea were infinite. Quantized tori violating this condition will not support regular states. Instead, chaotic states will flood these regions, see Fig. 1(a). In terms of dynamics we find that wave packets started in the chaotic sea progressively flood the island as time evolves. Partial and even complete flooding is possible, depending on system properties. These findings are relevant for islands surrounded by a large chaotic sea. We numerically demonstrate the flooding and the disappearance of regular states for the important case of island chains. In typical Hamiltonian systems they appear around any regular island. On larger scales they are relevant for Hamiltonian ratchets [5], the kicked rotor with accelerator modes [6], and the experimentally [7,8,9] and theoretically [10] studied kicked atom systems. The flooding of regular islands by chaotic states is a new quantum signature of a classically mixed phase space. This phenomenon shows that not only local phasespace structures, but also global properties ...