The structural perfection of gadolinium gallium garnet (GGG) substrates was shown to have a strong effect on the ferromagnetic resonance (FMR) linewidth in the yttrium iron garnet (YIG) epilayers. The most detrimental imperfections are dislocation pileups and iridium inclusions. A substrate preparation procedure was proposed which ensures the growth of YIG layers 4.7–5 μm in thickness, with an FMR linewidth of 24–40 A/m. YIG layers up to 70 μm in thickness were grown using lattice-matched iron garnet buffer layers.