The Craft of Writing (CoW) is an intervention aimed at improving the writing skills, writing self-efficacy and writing creativity (ideation) of primary school pupils by developing teachers as 'writers' and improving their own writing practice and their teaching of writing. This intervention was delivered to Year 5 pupils between June 2018 and July 2019 by the University of Exeter, the Open University and Arvon, a creative writing charity. This evaluation was designed to estimate the effect of participating in the Craft of Writing (CoW) over the course of one school year on pupils' writing skills. The CoW is an intervention to develop teachers as writers, thereby improving their own writing practice and their teaching of writing (more details below in the Intervention section). This trial was designed as a two-arm clustered randomised trial with randomisation occurring at the level of the school. This level of randomisation was selected since entire classes participate in the programme and thus the risk of contamination within schools is very high. The two arms were: (1) participation in the CoW (treatment); and (2) business-as-usual (control). Blocking was used in the randomisation to improve cross-arm comparability of schools, to improve precision of estimates, and to allow schools that sign up early to receive their allocation sooner than they otherwise would (this is important because of the nature of the intervention requiring as much notice of allocation as possible to be given to teachers, given it requires activity outside of their normal working hours; see the Intervention section for more details). Initially, the authors had planned to look at the long-term effects of participating in the CoW after one further year, looking at results from the end of KS2 national curriculum tests in English grammar, punctuation and spelling. [This report was produced with the Behavioural Insights Team of the University College London (UCL) (United Kingdom), Institute of Education (IOE). The Royal Society of Arts (RSA) contributed to funding for this evaluation.]