This book introduces optical wavelength conversion materials using luminescent lanthanide complexes. Their optical wavelength conversion properties are applied to color-tuning LED materials, UV photon-energy conversion for solar cells, and plant growth acceleration for future green-house application. In this book, the fundamental wavelength-conversion mechanism, recent trends and advanced research progress of optical wavelength conversion materials are explained and discussed using historical and recent academic reports. Furthermore, advanced future applications using luminescent lanthanide complexes,  are introduced.