In this chapter, we will discuss the theory of heat pipe with an approach that our readers have no knowledge of advanced mathematics, physics, and heat pipe. We cover the basic science and technology behind the heat pipe. Whenever we had to refer to basic knowledge of physics, fluid mechanics, and gas dynamics, Wiki site or very basic physics books to give the reader some general idea of specific topics of discussion in the particular section of this chapter along with heat pipe science were utilized. This section covers the fundamental theory behind the heat pipe based on different research papers and books available at present time in order to open a clear path for reader to design and fabricate their required heat pipe within their applications.