This book explores terahertz time domain spectroscopic characterisation of six different aminopyridine based organic materials. It also introduces two novel terahertz wave generating organic nonlinear optical crystals and makes a detailed comparison of its efficiency with the well known inorganic material GaP. This book will be a good read for those who pursue research in the field of nonlinear optics, more specifically for those who work on terahertz wave generation from nonlinear optical crystals. This book provides a short simple explanation of the process of terahertz wave generation from a nonlinear optical crystal via optical rectification along with a short, general introduction to nonlinear optics. This book explores terahertz time domain spectroscopic characterisation of six different aminopyridine based organic materials. It also introduces two novel terahertz wave generating organic nonlinear optical crystals and makes a detailed comparison of its efficiency with the well known inorganic material GaP. This book will be a good read for those who pursue research in the field of nonlinear optics, more specifically for those who work on terahertz wave generation from nonlinear optical crystals. This book provides a short simple explanation of the process of terahertz wave generation from a nonlinear optical crystal via optical rectification along with a short, general introduction to nonlinear optics.