The main intensity limitations in future hadron or positron rings are expected to come from unconventional loss mechanisms. First, we will present a review of the single bunch collective effects driven by electron cloud. The wake field generated by an electron cloud can lead to transverse beam instability and loss over a few turns. The electron cloud wake field is highly unconventional, since the shape of the wake depends on the cloud distribution, and therefore on which part of a passing bunch

more »

... is generating it. This feature leads to the definition of a generalized impedance, which has a double frequency dependence and can be used in TMCI calculations for the accurate estimation of the instability threshold. Space charge and conventional broad band impedance can also interplay with the electron cloud to trigger stronger instabilities. The second part of the paper will address the features of collective head-tail motion in a barrier bucket under the action of a transverse broad band impedance. Similarities and differences with regular and strong head tail instability for a conventional bunch shape will be highlighted.