In this talk, we explore canonical systems in the viewpoint of spectral theory. First we see that canonical systems are (the most) natural generalization of eigenvalue equations for Schrodinger operators, Jacobi matrices (so far, Sturm-Liouville operators) and Dirac operators. This generalization can be performed without changing their spectra via Weyl-Titchmarsh m-functions. Moreover, by introducing the time variable, we try extend Kortweq-de Vries (KdV) flows to canonical system flows via so-called zero-curvature equation in two natural ways.