In this book we have carried out detailed ab-initio studies to determine and analyze the structural, electronic, magnetic, optical and thermoelectric properties of rare-earth-based double perovskites (DPs). We carried out our calculations using the augmented plane wave method (FP-LAPW), which is based on density functional theory (DFT) and is integrated into the Wien2K calculation program. The results obtained were compared with experimental data from the literature and other results from theoretical research.