Vibrational analysis of rotating machinery is able to identify a large number of system ills. Shaft bow, shaft unbalance and coupling misalignments make up the major portion of the observed vibrational frequency spectra of rotating machinery. These vibrational spectra can be used to determine the type of rotating system abnormality, the degree of misalignment and the rate of alignment degradation. The proper joining of rotating machinery is a critical issue with high power and a high speed equipments. Couplings used to join machinery have a broad spectrum of characteristics and specific purposes. Couplings are used to compensate for axial, lateral and angular misalignments. In addition, couplings may exhibit constant or non-constant input versus output speed relationships. The importance of mating a driving and driven piece of equipment with the properly designed coupling is paramount. In the following report the author discusses numerous types of couplings, the behavioral characteristics of couplings and the considerations which must be taken into account when selecting a coupling for a rotating system. The analysis which follows list the various alignment discrepancies which can be detected by vibrational analysis and the requirements for non-constant and constant velocity characteristics of couplings. In addition, numerous types of couplings are catalogued and discussed. Theses. (FR)