abstract: We present a theory of metric viscosity solutions which encompasses a large class of Hamiltonians. We consider time dependent problems \[\label{a111} \partial_tu+H(t,x,u,\vert\nabla u\vert)=0,\quad\mbox{in}\,\,(0,T)\times \Omega, \] \begin{equation}\label{a111BC} \left\{ \begin{array}{ll} & u(t,x)=f(t,x)\quad\mbox{on}\,\,(0,T)\times \partial\Omega, \\ & u(0,x)=g(x)\quad\mbox{on}\,\,\Omega, \end{array} \right. \end{equation} and stationary equations. We prove a range of comparison and existence results that apply to a wide range of equations and we present a sample of techniques that can apply in other cases. (This talk is based on a joint work with A. Swiech).