The design of high performance `factories', large hadron colliders and synchrotron light sources calls for a large number of high intensity bunches. This imposes feedback systems and a tight impedance budget to control conventional instabilities, some of which are differently emphasised depending on the ongoing evolution of beam parameters. Ion trapping, for example, is no longer reported as a problem for the new generation of very low emittance electron storage rings. However new mechanisms appear, such as the fast ion instability for electron beams and the build-up of electron clouds for positron or proton beams. We review these new manifestations of collective phenomena, essentially related to the single-pass interaction of a bunch train with foreign or `gaijin' particles, and discuss their dependence on several machine parameters such as bunch intensity and spacing. We also summarise possible cures and positive as well as negative experimental evidence in existing accelerators
