The unstoppable tendency toward Information Systems Outsourcing contrasts with the lack of success in many of these contracts, which makes it necessary to study the factors determining that success. The present paper proposes a set of key factors to achieve IS outsourcing success and also; it establishes a number of measures to value the degree of success reached by firms at outsourcing and verifies the existence of a positive relationship between “being successful at Information Systems Outsourcing” and “valuing success factors positively”. We have validated all of this through a survey carried out among the Information Systems managers of the largest Spanish firms.