Apraxia of Speech (AoS) is a neurogenic speech disorder. A wide variety of behavioural methods have been developed to treat AoS. Various therapy programmes use musical elements to improve speech production. A unique therapy programme combining elements of speech therapy and music therapy is called Speech-Music Therapy for Aphasia (SMTA). In clinical practice, patients with AoS have experienced positive outcomes of SMTA; however, there was no evidence of this treatment’s effectiveness. This thesis addresses that topic. First, a description of the literature on speech processing in healthy speakers and patients with AoS is described, followed by the treatment of AoS and various characteristics of music that may be involved in this treatment. Next, an instrument to evaluate the effect of SMTA has been developed to assess the effects of SMTA; the Modified Diadochokinesis Test (MDT) in which patients are requested to repeat syllables. The MDT has adequate psychometric properties, implying that it can be used to measure changes in speech motor performance after AoS treatment. Finally, an efficacy study of SMTA was performed in a small group of patients with AoS. Verbal communication in daily life improved in all participants and this improvement remained stable after treatment stopped. The general discussion chapter of this thesis explains the working mechanism of SMTA. SMTA provides an external musical frame directed to the dynamic process of speech motor control. The musical parameters melody, rhythm, meter, tempo and dynamics are related to a particular level of speech production: speech motor planning.