The quality of software is a set of qualities that characterize it and that determine their utility and existence. The quality is synonymous of efficiency, flexibility, correction, trustworthiness, portability, usability, security and integrity. The quality of software is measurable and varies from a system to another one or from a program to another one. A software elaborated for the control of spaceships must be reliable at the "zero errors level"; a software made to execute a single time does not require the same quality level; whereas a product of software to be exploded during a long period (10 years or more), needs to be reliable and flexible to diminish the costs of maintenance and improvement during the time of operation. The quality of software can be moderate after elaborated the product. But this can be very expensive if problems are detected derives two from imperfections in the design, reason why is essential to consider the obtaining of the quality and its control during all the stages of the service life of software.