It is well established that magnetic components may be reduced in size by operating at high frequency. Miniaturisation of magnetic components is ideally suited to microelectronics technologies such as thick film which lend themselves to planar geometries. This paper describes new analytical models which predict inductance and frequency dependent eddy current losses in magnetic substrates. Prototype devices were fabricated by a thick film process; with 4 layers of conductors on a single ferrite substrate and in sandwich configuration, consisting of conductors between ferrite slabs. The prototype devices were tested in the frequency range 10 kHz-100 MHz. The measurements confirm the validity of the analytical models. Simulation with Finite Element Analysis was employed to identify different sources of losses: eddy current losses in ferrite substrates, proximity effect losses in conductors and dielectric losses.