We have developed optical pumping techniques enabling us to restructure the inhomogeneous absorption profile of rare earth ion doped crystals such that we can create frequency-sharp artificial absorption lines and transmission window. In a manner analogous to an electronic a bandpass filter a (transmission window) will slow down electro-magnetic pulses propagating through the transmission window. If the filter is narrow it will slow down the speed of propagation significantly. For our transmission windows the speed of light typically reduces by 3-5 orders of magnitude. The spectral width and frequency positions of our transmission windows can be controlled by external electric fields, which open for interesting opportunities. For example changing the transmission window frequency with changes the speed of light in the material. Specific projects utilising the slow light phenomenon are laser frequency stabilization and medical imaging.