With English-medium instruction (EMI) as a growing trend worldwide, a major concern is whether teaching quality is affected in the shift from L1 to English. Taking a broad view of effective EMI teaching, which goes beyond language proficiency, this chapter analyzes two parallel lectures delivered in the L1 and in English by the same lecturer, exemplary of good teaching. Combining qualitative and quantitative methods, this study explores what makes effective EMI teaching according to stakeholders' perspectives and whether it changes from L1 to EMI. By comparing lecturer's discourse and lecturing style (personal/impersonal, interactive/monologic, formal/informal), students' satisfaction, accounts of classroom practices, and participants' views of what makes effective lecturing, this study provides a detailed view of the elements that make up effective EMI lecturing in order to derive implications for EMI training.