The need to adapt the length of the working day to meet the interests of both firms and workers is a reality that has imposed itself on work relationships throughout the whole of the European social space. The maximum number of work hours per day set out in the current labour regulations of the Member States is completed with part-time work. Faced with the need to bring the national legislations on this matter into line, the European Union issued the Council's Directive 1997/81, of 15 December 1997, concerning the framework agreement on part-time work drawn up by UNICE, CEEP and ETUC. Spain has adapted its regulations to the requirements and interests of both workers and employers by introducing corrections and modifications into each and every one of the labour reforms that have been brought in since the Workers' Statute was adopted in 1980. This study deals with the regulatory process and the legal system concerning part-time work currently applicable in Spain and came into force with the enactment of the Real Decreto-Ley 5/2001, of 2 March.