A new fully dynamical scheme for constant temperature and pressure simulation of particle systems, based on a modification of the Nosé thermostat. A mechanical formulation for simultaneous control of temperature and pressure is also presented. This approach simplifies the construction of symplectic methods while providing a more intuitive perspective on the nature of controlled variable molecular dynamics. Moreover, the described method is Gallilean-invariant, hence angular momentum preserving, and the per-timestep simulation costs are similar to the per-timestep costs of microcanonical (N, V, E) simulation. The scheme is suited to molecular simulation and to large scale particle models of fluids such as smooth particle hydrodynamics and dissipative particle models.