This is the English version of a CNES technical note from 10 October 1983 (see DOI: 10.13140/RG.2.1.2100.8880) The goal of this note is to define and study multipactor effect that may be responsible of failure or even destruction of power radiofrequency equipment in vacuum, particularly satellite transmitters, output circuits and antennas. The knowledge of the conditions for multipactor effect is essential for satellite design, particularly those transmitting high power such as direct television and synthetic aperture radars (SAR). This study has been done in part to support EOPO (Earth Observation Program Office) for the SAR satellite ERS1. The theoretical study is based on previous simple and empirical studies. It shows the limitations of these theories and of the classical definition of multipactor effect. A more rigorous analytical study is proposed. By applying simple physical criteria (stability or instability, limit conditions, …) more interesting results are obtained without requiring empirical values. Then, after comparing theoretical results with published or obtained experimental results, this note proposes directions for further work and a better knowledge of the phenomenon.