It is widely recognized that teacher training affects the fidelity with which evidence-based substance use prevention curricula are implemented. This article presents the results of a 2005 survey of teachers from a nationally representative sample of 1721 public middle schools in the US (78.1% response rate).It measured fidelity along two dimensions (adherence and dose) and also assessed the number of hours, recency, and perceived effectiveness of teachers’ training, as well as the degree to which adherence was emphasized during training. Among teachers using evidence-based curricula, 35.3% reported following their curriculum guide very closely. The average proportion of lessons taught was 64.9%, and only 30.2% of teachers taught all the lessons in their curriculum. Analyses revealed that teachers whose training emphasized adherence were five times as likely to be more adherent. The article presents recommendations for training-related factors that may increase fidelity of implementation.