Electronic phase separation and spin glass behavior were certainly the new areas in perovskite oxides which have recently been established experimentally. The main purpose of this book is to introduce the phenomenon and physics of complex magnetism (phase separation, spin glass, glassy magnetism, frustrations etc) in perovskite manganates and cobaltates via an experimental approach. A variety of magnetic and electronic properties manifest in hole-doped perovskite oxides depending on the various factors such as the A-site cation size and size-disorder effect as well as external factors (pressure, temperature, magnetic field etc). The physical properties of these oxides can be tuned by external factors or by chemical means. Such phenomena are manifested in polycrystals and single crystalline samples as well as in thin film. This book has revealed the presence of complex glassy magnetism in both perovskites, albeit of different varieties.