The book "Basic Concepts of String Theory" offers a comprehensive introduction to string theory at an intermediate level. It is aimed at readers who wish to prepare for research in this field. The content is didactically and independently structured, making it a suitable textbook. In addition to the fundamental concepts, advanced topics such as conformal field theory, superstrings, and string dualities are also covered. It is important to note that the book does not address applications in black hole physics or cosmology, nor does it cover string theory at finite temperatures. At the end of each chapter, there are references that support the reader in further studies.