Providing instructional explanations is a core component of effective instruction and an important teaching skill. Teaching skills are generally regarded as learnable, and teacher education programs aim to improve teachers' professional competences. In this study, we analyze to what extent explaining skills can be fostered during teacher education at university by means of a specific training module. We designed a training (university module) for prospective economics teachers at vocational schools (candidates in a Master's teaching program). By means of videotaped simulated interactions at two measurement points, we analyzed the development of teacher candidates' explaining skills. Teacher candidates were asked to explain the neoclassical supply and demand model (treatment group: n = 48; control group: n = 30) to an actor playing the role of a school student. The quality of the explanations was operationalized in respect of five aspects of successful explanations, which were derived from a literature review: (1) Content, (2) Student-teacher interaction, (3) Process structure, (4) Representation, and (5) Language. The results show that there was a treatment effect on the development of the Process structure aspect, while Student-teacher interaction appeared to develop "naturally" through experience, regardless of participation in the training. The quality aspects Content, Representation, and Language appeared stable over time. Hence, the findings show that some aspects of explaining skills are learnable even in a short training module. Learning effects are attributable partly to the instructional input received and partly to repeated practice. Both imply the importance of further opportunities to practice instructional explanation in teacher education.