ABSTRACT: The authors review the formalism of effects and operations in order to demonstrate that this formalism is equivalent to the standard rules of quantum mechanics. D'Espagnat (1986) has studied an example where he finds a discrepancy between an analysis based on the standard quantum rule for calculating probabilities and an analysis that uses a particular operation taken from the work of Barchielli and his collaborators (1982). They use the formalism of effects and operations to explore and explain this discrepancy.