Training is a vital component of effective and quality intervention. Outcomes of instruction are intrinsically linked to the skill sets of instructors. While training is known to be the foundation of good instruction, there is sparse information about the repertoires of an effective trainer. In this chapter, we review the responsibilities associated with roles such as supervisor and trainer. Special attention is paid to the specific and targeted focus of training in the context of applied behavior analysis (ABA) intervention. We will discuss the competencies of a trainer, including the ability to identify core needed skills, adapting to varied trainee characteristics, working across instructional modalities, and using evidence-based practices to instill important competencies in trainees. We will also review the fundamental technical skills of training, including content knowledge, pinpointing learning objectives, and measuring training outcomes. Additionally, we will discuss the most important interpersonal skills required for success in the role, including being flexible, compassionate, patient, and inclusive. Core skills required for success in training will be highlighted, including professionalism and communication. The distinction between basic and masterful training repertoires will be noted. Scenarios are presented to identify technical and interpersonal component skills that facilitate successful training.