While the benefits of breastfeeding are well established, few Scottish women exclusively breastfeed, as recommended nationally and internationally. Breastfeeding peer support can help mothers to breastfeed for longer, but the training peer supporters receive is variable and few studies have measured the effectiveness of peer supporter training. This study aimed to compare mothers' attitudes towards and knowledge of breastfeeding before and after undertaking Breastfeeding Network (BfN) peer supporter training. This study used a quasi-experimental design to investigate levels of breastfeeding knowledge and attitudes in 28 women, immediately before and after the BfN peer supporter (Helper) training programme. Data were collected using the Infant Feeding Questionnaire. Mothers had high baseline scores for attitude (88.8%) and knowledge (76.4%). After training 78% of mothers improved their attitude scores. The mean difference in attitude scores was 4.9%, which was statistically significant (t = 4.44, P < 0.001). Ninety-six per cent of mothers had increased their knowledge scores. The mean difference in knowledge scores was 10.4%, which was statistically significant (t = 6.25, P < 0.001). This study provides evidence that the BfN's accredited peer support training can improve both breastfeeding knowledge and attitudes of breastfeeding mothers. Peer supporters who have undertaken this training are therefore suitably qualified to provide mothers with the support and information required to make informed choices in breastfeeding and to contribute to effective support and promotion of breastfeeding as suggested in the National Institute for Health and Clinical Excellence guideline on maternal and infant nutrition (PH11) and Scottish Government's Maternal and Infant Nutrition: A Framework for Action (2011).