We present comprehensive case studies on trapping of light in plasmonic waveguides, including the metal-insulator-metal (MIM) and insulator-metal-insulator (IMI) waveguides. Due to the geometrical symmetry, the guided modes are classified into the anti-symmetric and symmetric modes. For the lossless case, where the relative electric permittivity of metal (epsilon(m)) and dielectric (epsilon(d)) are purely real, we define rho as rho = -epsilon(m)/epsilon(d). It is shown that trapping of light occurs in the following cases: the anti-symmetric mode in the MIM waveguide with 1 < rho < 1.28, the symmetric mode in the MIM waveguide with rho <<1, and the symmetric mode in the IMI waveguide with rho <1 . The physical interpretation reveals that these conditions are closely connected with the field distributions in the core and the cladding. Various mode properties such as the number of supported modes and the core width for the mode cut off are also presented.