It is mainly since the early 1920*8 that the rotational spectra of diatomic molecules have been investigated. During this time it has become clear how deeply rooted are the connections between these spectra and the internal structure of the molecules and how important is the information they give about molecular architecture. It has become evident that these spectra are, say, coded messages from the molecules in secret writing and that to decipher these messages it is necessary to determine experimentally the position and intensity of the spectral lines and to establish the theoretical laws governing the spectra. Although the fundamental investigations about rotational spectra were published in the third and fourth decades of this century and the fundamental theory can be found in a number of excellent books, it seems appropriate and necessary to collate this material in the form of a unified system and to complete this system by adding new elements. This is especially true with respect to the formulas of intensity distribution, a systematic collection of which is in growing demand by physicists and astronomers. The author's aim is to meet this demand by the present book.