The Preschool Autism Communication Therapy (PACT) is the only treatment method for early autism to have shown a long-term effect on autism symptom developmental outcomes into middle childhood; as well as evidence of the mechanism through which these effects are produced. The PACT model is designed on the basis of developmental and transactional principles applied to autism, to form a relatively low intensity naturalistic intervention that can be flexibly applied in home or clinic. It works with parents, caregivers or teachers using video-feedback methods to enhance recognition and responsiveness to child communication and optimize child social communication development. There have to date been five replication studies of the core effect of this intervention on parent-child interaction, and four for the effect on child symptom outcomes. PACT has been subjected to the longest post-intervention follow-up to date in the field, showing a sustaining of its effects on child development. An adaptation has been shown effective in low and medium income country settings, and a related pre-emptive intervention effective with infants at autism risk. Training on the PACT model is available through a mixture of online e-learning, face-to-face training and supervised practice teaching (http://research.bmh.manchester.ac.uk/pact/TrainingPACT/).