This chapter provides a description of the method used in variationist sociolinguistics, called the variationist approach. The use of corpora is intrinsically linked to the variationist approach. Since its inception, this approach was mainly used to describe phonetic phenomena, and its application to the study of morphosyntactic structures has been controversial. Due to the nature of sociolinguistics as a discipline, this method puts emphasis on the interaction between performance and linguistic competence, in contrast with generative grammar, which focuses on linguistic competence. This chapter explores the utilization of the variationist approach for the study of morphosyntactic structures, as well as its similarities with the method used in recent 138quantitative studies within the generative framework. Section 2 deals with the fundamental principles of the variationist model. It also addresses questions which, although controversial, are frequently bound to the study of morphosyntactic variation, such as (i) whether it is possible to make linguistic generalizations from just one individual, (ii) whether the use of written corpora in sociolinguistics is appropriate or (iii) whether syntactic variation really exists. Section 3 compares corpus studies that focus on morphosyntactic variation from a sociolinguistic perspective with others that follow the generative tradition. Section 4 is devoted to a description of the methodology and tools used in variationist studies. Finally, section 5 presents the conclusions and provides a panoramic view of the future of the discipline.