A common early intervention approach for preschool children with language problems is parent-child interaction therapy (PCIT). PCIT has positive effects for children with expressive language problems. It appears that speech and language therapists (SLTs) conduct this therapy in many different ways. This might be because of the variety of approaches available, the diverse set of families SLTs work with or the different organizational structures. Understanding the critical components of PCIT would enable SLTs to map the variations that are implemented and researchers to evaluate the effects of such variation. This study aimed to identify the potentially critical components of PCIT based on the practical experience of SLTs and to identify SLTs' rationales for the way they structure PCIT. Both parameters are important for the long term goal, that is, to develop a framework that can be used to support practice. Semi-structured interviews were conducted with 10 SLTs who had at least one year experience in delivering PCIT with preschool children with language impairment. The interviews were transcribed and analysed, using thematic analysis. Analysis of the SLT interview data identified four potentially critical components that underpin the teaching of strategies to parents: parents' engagement, parents' understanding, parents' reflection and therapists' skills. SLTs suggested that all four components are needed for the successful delivery of PCIT. The reasons that SLTs give for the way in which they structure PCIT are mainly based on organizational constraints, family needs and practicalities. SLTs consider PCIT to be valuable but challenging to implement. A framework that makes explicit these components may be beneficial to support practice.