Speakers who acquire a foreign language (L2) in adolescence or later often experience difficulties in the production of non-native sounds. Their L2 speech is easily identified by native speakers as having a foreign accent. This thesis focuses on second-language learners' production difficulties. It attempts, first, to understand the causes of these difficulties with a particular attention paid to L2 perception and native (L1) production. Second, it tests the effectiveness of a new visual articulatory-feedback training method that aimed at enabling L2 speakers to improve their L2 pronunciation. Last, it explores the interaction between the production of native and non-native sounds. In particular, it examines the role of within- and between-speaker variability in L1 production in predicting L2 production and perception performance, and it assesses the effects of learning to produce non-native sounds on L1 production.