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Best practice recommendations on autism spectrum disorders (ASD) were published in France in 2018. These reaffirm the structuring of the diagnostic pathway for ASDs by recommending that third-line structures focus on the diagnosis of “complex cases.” The objective of this work was to situate the activity of a third-line unit, and to clarify the definition of complex diagnoses. Mixed methods were used thanks to two focus groups conducted with members of the Udela and an epidemiological study of 105 patient files received during the 2017–2018 academic year. The study of the files was first carried out on the basis of criteria defining a complex situation according to the recommendations, then these criteria were enriched and made more specific based on the experience of professionals collected by the focus groups.