Over the last several years, the use of nuclear hyperpolarization techniques in nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) has increased exponentially. Since the signal sensitivity can be enhanced by orders of magnitude compared to that in the normal conditions, nuclear hyperpolarization, when applied to NMR/MRI studies of chemical/biological interest, could enable us to observe what we have so far not been able to, and thereby bring us into a new realm of science. This book focuses one of several ways to attain nuclear hyperpolarization, namely, dynamic nuclear polarization (DNP) using electron spins in the photo-excited triplet state, addressing its basic principles, discussing the elementary processes of spin-polarization transportation behind the buildup behavior, and presenting practical experimental tips. Thus, this book should be especially useful to graduate students in physics and chemistry studying nuclear hyperpolarization, and anyone who may be considering utilizing nuclear hyperpolarization for NMR/MRI in chemical, biological, and medical sciences.