Three criteria for diagnosing specific language impairment (SLI) are defined in the literature: exclusionary definitions, inclusionary definitions, and qualitative markers. However, the assessment and diagnosis of this complex impairment in clinical practice can be challenging. The aims of this research were twofold: (1) to make explicit the current assessment procedures which speech and language therapists (SLTs) use when they diagnose children with SLI in Ireland, and (2) to explore SLT's perceptions and experiences of the process of diagnosing children with SLI. Both quantitative and qualitative methods were used in this study. A survey of 199 SLTs in Ireland was carried out to address aim one and a focus group was conducted to address the second aim. The findings were that SLTs use exclusionary and inclusionary definitions of SLI, and qualitative markers when assessing children who they suspect may have SLI. SLTs in Ireland consider a diagnosis of SLI when children present with a verbal-performance discrepancy, a positive family history, word finding difficulties and lack of progress in therapy. The results of the qualitative strand indicated that assessment and diagnosis of children with SLI is a complex, time-consuming process involving other professionals. What emerged was a complex picture with tensions between therapists' professional judgements, policy, evidence-based practice and resources.