Metal layers one atom in thickness are the ultimate in ultrathin films. They are the object of intensive study. The preparation, characterization and properties of metal monolayers are reviewed. This field of research provides a sort of common meeting ground for clean-surface physics, chemisorption, and thin-film and surface metallurgy. The topics discussed include: the kinetics and thermodynamics of monolayer formation, monolayer structures and ‘two-dimensional’ phase transformations, desorption and binding energetics, electronic properties and chemical reactivity.