Background: There are few economic evaluations of speech and language interventions. Such work requires underpinning by an accurate estimate of the costs of the intervention. This study seeks to address some of the complexities of this task by applying existing approaches of cost estimation to interventions described in published effectiveness studies. Aims: The study has two aims: to identify a method of estimating unit costs based on the principle of long-run marginal opportunity costs; and to illustrate the challenges in estimating unit costs for speech and language interventions. Method & Procedures: Descriptions of interventions were extracted from eight papers and combined with information on the unit cost of speech and language therapists to identify information requirements for a full-cost estimation of an intervention. Outcomes & Results: Four challenges were found relating to the level of detail about the therapists, the participants, the scope of activities and parents. Different assumptions made about any of these elements will have a marked effect on the cost of the intervention. Conclusions & Implications: Nationally applicable unit cost data for speech and language therapists can be used as a reference point, but sufficient descriptive data about delivery and receipt of the intervention are key to accuracy. (Contains 1 note and 3 tables.)