This book introduces the current state of research on dark energy. It consists of three parts. The first part is for preliminary knowledge, including general relativity, modern cosmology, etc. The second part reviews major theoretical ideas and models of dark energy. The third part reviews some observational and numerical works. The aim of this book is to provide a sufficient level of understanding of dark energy problems, so that the reader can both get familiar with this area quickly and also be prepared to tackle the scientific literature on this subject. It will be useful for graduate students and researchers who are interested in dark energy.