People often use the word 'competence' without understanding what it means even when it is vital for safety. This paper examines common definitions of competence to identify the individual components and understand the principles underlying the specification and assessment process. Safety management must facilitate the achievement and maintenance of competence for those developing and operating safety-related systems. The paper examines the theory and principles underlying the attainment and maintenance of competence providing a framework for a discussion on competence assurance. The specification of competence criteria is an important safety management activity and these are unique for different systems. This paper describes how competence criteria can be specified and assessed for safety-related systems. Safety assurance is ultimately based upon the competence of the people involved hence competence evidence is essential for the validity of any safety claim. The paper examines common safety assurance issues associated with competence and some suggestions are made on how to improve the validity of safety claims based upon competence.