This study aims to explore whether and why speech and language therapists (SLTs) use informal sources to guide their intervention decisions for children with developmental language delay (DLD). A qualitative study was conducted. Sixteen Turkish SLTs participated in this study. The methods of interviewing, participant observation and audio diary were used. An inductive thematic approach was taken to analyse the data. Five main themes were revealed from the data: 'The lack of trust in the formal standardised tests and sources', 'experience over theory', 'trial-error, 'other professionals' opinion' and 'instinct' showed that the SLTs value variety of sources and their cumulative experience when making an intervention decision. While evidence-based practice (EBP) has been increasingly used and encouraged in the SLT profession, this study shows that informal sources play a significant role in SLTs' clinical reasoning. This study sheds light on the behind factors of relying on informal sources to make intervention decisions. The role and dynamic of using informal sources could help understand the value and risks of this approach in clinical reasoning.