A method based on vector spherical mode expansion (VSME) for solving radiation and scattering problems is developed. Combined with numerical methods such as MoM or FDTD, three dimensional electric and magnetic current sources are theoretically analysed according to image theory and equivalence principles. The electric and magnetic VSME coefficients introduced are written in terms of the electric and magnetic current densities with space sinusoidal and Legendre functions. By using these coefficients and the correct choice of the co-ordinates, the electromagnetic fields can be constructed with a minimum number of modes.