Background: Atypical speech and language development is one of the most common developmental difficulties in young children. However, which clinical signs characterize atypical speech-language development at what age is not clear. Aim: To achieve a national and valid consensus on clinical signs and red flags (i.e. most urgent clinical signs) for atypical speech-language development in children from 1 to 6 years of age. Methods & Procedures: A two-round Delphi study in the Netherlands with a national expert panel (n = 24) of speech and language therapists was conducted. The panel members responded to web-based questionnaires addressing clinical signs. Consensus was defined as = 70% of the experts agreeing on an issue. Outcomes & Results: The first round resulted in a list of 161 characteristics of atypical speech and language development. The second round led to agreement on 124 clinical signs and 34 red flags. Conclusions & Implications: Dutch national consensus concerns 17-23 clinical signs per age year for the description of an atypical speech-language development in young children and three to 10 characteristics per age year being red flags for atypical speech-language development. This consensus contributes to early identification and diagnosis of children with atypical speech-language development, awareness and research.