The mechanistic understanding of peptide self-assembly mediated by metal ions into amyloids or other structures has gained a lot of interest, mainly due to their importance in several neurodegenerative diseases. The use of short, easy-to-handle peptides has contributed to a better knowledge of structures and mechanisms that are also relevant for the native and longer peptides involved in the neurodegenerative diseases. Here, we review the results obtained with such "model systems" with the aim to identify and discuss fundamental parameters determining the self-aggregation with a special focus on the role of metal ions in this process.