In this note, we present criteria for marginal stability and marginal instability of switched systems. For switched nonlinear systems, we prove that uniform stability is equivalent to the existence of a common weak Lyapunov function (CWLF) that is generally not continuous. For switched linear systems, we present a unified treatment for marginal stability and marginal instability for both continuous-time and discrete-time switched systems. In particular, we prove that any marginally stable system admits a norm as a CWLF. By exploiting the largest invariant set contained in a polyhedron, several insightful algebraic characteristics are revealed for marginal stability and marginal instability.