The mechanics of large-strain deformation in cutting of metals is discussed, primarily from viewpoint of recent developments in in situ analysis of plastic flow, and microstructure characterization. It is shown that a broad range of deformation parameters can be accessed in chip formation strains of 1-10, strain rates of 10-105/s and temperatures up to 0.7Tm - and controlled. This range is far wider than achievable by any other single-stage, severe plastic deformation (SPD) process. The resulting extreme deformation conditions produce a rich variety of microstructures in the chip, which in turn are inherited on machined surface. Four principal types of chip formation – continuous, shear-localized, segmented and mushroom-type – as elucidated first by Nakayama , are utilized to emphasize the diverse unsteady plastic flow phenomena that prevail in cutting. These chip types are intimately connected with the underlying flow, each arising from a distinct mode and triggered by an instability phenomenon. The role of plastic flow instabilities such as shear banding, buckling and fracture in mediating unsteady flow modes is expounded upon. For example, sinuous flow is shown to be the reason why gummy (highly strain-hardening) metals, although relatively soft, are so difficult to cut. Synthesizing the various observations, a hypothesis is put forth that it is the stability of flow modes that determines the mechanics of cutting. This leads to a flow-stability phase diagram that could provide a framework for predicting chip types and process attributes.