Computational thinking (CT) as one of the 21st century skills enters early years education. This paper aims to study the worldwide tendencies of teaching CT through computing in primary education and primary school teachers' understanding of CT. A survey of 52 countries has been performed and complemented by a qualitative study of 15 countries. In order to identify teachers' understanding-level of CT and its integration approach in the class activities, a case study of 110 in-service teachers from 6 countries has been performed. The implications of the research results may be useful for primary school educators, educational initiatives, government authorities, policy makers, e-learning system and content developers dealing with support for teachers aiming to improve their CT professional development qualification.