The physics of sawtooth oscillations in plasmas produced by the Alcator C-Mod machine is discussed by identifying the regimes that characterize these oscillations, which occur in the central value of the electron temperature, as measured by X ray and/or ECE diagnostics. Sawteeth are generally present in ohmic as well as in RF heated plasmas. It is shown by numerical analysis that these plasmas are stable to ideal internal, pressure driven modes. Hence, the modes responsible for the crash phase of the sawteeth have to be resistive in nature. This is confirmed by numerical computations using a resistive code. The role of energetic ions, produced by ion cyclotron heating, is also discussed.