The electromagnetic scattering from arbitrarily shaped 3-D cavities with coated walls is presented. In this formulation, the cavity is divided into smaller sections. Each section is individually analyzed with a boundary integral equation, followed by the standard moment method procedure to arrive at an admittance matrix for each section. These admittance matrices for different sections are then cascaded using a connection scheme based on microwave network theory. Once the admittance matrix of the whole cavity is obtained, the electric field on the aperture and thus the backscattered field can be obtained by the generalized network formulation. Triangular patch basis functions are used to provide easy mesh generation using standard CAD tools. To model wall treatment, the impedance boundary condition is applied. The numerical results generated by the present approach show good agreement with the modal solutions for regular geometries. An example for a cavity with coated walls is also presented.