Chapter 2 takes up the issues of validity that were introduced in Chapter 1. We describe the four main types of validity described in the intervention literature and then proceed to review two of these (internal and external validity) in detail. We propose seven features to incorporate into a SCED to address the important threats to internal validity. The section on external validity discusses the two central features that will bolster external validity and generality: replication and generalisation (including social validity). We then provide a worked example from the neurorehabilitation literature (Broach & Dattilo, 2001) to demonstrate how their study addressed the four main types of validity. The chapter then overviews a selection of five published critical appraisal scales to measure the scientific quality of a SCED. The chapter concludes by presenting the SCRIBE reporting guideline, a consensus document developed by an international group of SCED experts. The 26-item SCRIBE checklist provides an invaluable guide for authors to publish reports which are accurate, clear, complete, and transparent.