Coordination bonds are most often reversible. Using them to crosslink polymers into for example hydrogels can therefore be expected to result in reversible crosslinks, which in turn provides self-healing properties. Polyphenols such as the catechol group of the special amino acid DOPA (L-3,4-dihydroxyphenylalanine), which is used in mussel byssal threads to form a self-healing coating and for adhesion, are excellent coordinators of hard metal ions like iron(III). We will discuss the use of polyphenols and metal crosslinking to form self-healing hydrogels. We also treat less specific effects of metal ions on hydrogel gel mechanical properties by describing Hofmeister effects of cations on hydrogel rheological properties.