Events associated with aversive or rewarding outcomes are prioritized in memory. This memory boost is commonly attributed to the elicited affective response, closely linked to noradrenergic and dopaminergic modulation of hippocampal plasticity. Here, we review and compare this ‘affect’ mechanism to an additional, recently discovered, ‘prediction’ mechanism whereby memories are strengthened by the extent to which outcomes deviate from expectations, that is, by prediction errors. The mnemonic impact of prediction errors is separate from the affective outcome itself and has a distinct neural signature. While both routes enhance memory, these mechanisms are linked to different, and sometimes opposing, predictions for memory integration. We discuss new findings that highlight mechanisms by which emotional events strengthen, integrate, and segment memory.