Three principal agents of metamorphism are:

1. Heat (temperature): drives recrystallisation and mineral reactions; increases with burial depth and proximity to magma
2. Pressure: confining pressure (lithostatic) increases with depth; directed pressure (stress) causes foliation and lineation
3. Chemically active fluids: water, CO₂, and dissolved ions catalyse mineral reactions, enable mass transfer, and are essential for many metamorphic reactions

All three act together during regional and contact metamorphism.