Komatiites are ultra-hot ultramafic lavas, largely restricted to the Archaean. They represent an extreme endmember of terrestrial magmatism and challenge our understanding of how mantle melting operates. We briefly introduce this compositionally diverse group of lavas and critically evaluate constraints on their formation. Despite evidence for moderate water contents in some komatiites, the vast majority require an unusually hot mantle source and probably formed by critical melting in dry or ‘damp’ plumes. The low concentrations of incompatible trace elements in most komatiites cannot be explained by residual phases rich in these elements and instead reflect high degrees of partial melting. Constraining the melting pressures of komatiites is complicated by a lack of robust constraints. However, high MgO contents, high degrees of partial melting, and evidence of residual garnet in the formation of Al-depleted komatiites indicate that melting began at considerable depth in the upper mantle, if not within the lower mantle. We combine these constraints to present models for komatiite formation. Al-depleted komatiites are high pressure melts of fertile mantle; they segregated from sources containing residual garnet at pressures >7 GPa and possibly >10 GPa. Al-undepleted komatiites segregated at lower pressures and/or after reaching higher degrees of partial melting. They came from a depleted source that may have formed by low degrees of hydrous melting in the mantle transition zone. Al-enriched, or Ti-depleted komatiites originated from extremely depleted sources. Their melting pressures are difficult to ascertain, but evidence from the Commondale komatiites suggest at least some formed at pressures >10 GPa. Ti-enriched komatiites and post-Archaean komatiites were produced by smaller degrees of melting of variably enriched or depleted sources, with melting conditions comparable to those of modern picrites.