This study focused on trainers' professional knowledge and skills. Forty-one aspects of trainer knowledge were identified from the empirical research literature and presented in an online questionnaire to 200 trainers, 253 trainees and 93 human resource development practitioners. The respondents rated the importance of each knowledge aspect for the professional activities of a trainer on a six-point Likert scale. The results indicate that subject-matter knowledge and communication techniques are considered vital for trainers, along with content-specific instructional knowledge such as specific training methods. Additionally, trainers are expected to provide clarity and structure, build relationships with trainees and create a constructive learning environment. Implications for future research and training practice are discussed.