This study investigates science teachers' understanding and teaching of complex systems. The field of complex systems is the study of how parts of a system give rise to its collective behaviors. Since the 1990s, scientific and educational agencies have advocated the importance of complex systems in science education. Despite this call for instructional emphasis in complex systems, recent studies have shown that students continue to have poor understanding of these systems. Current efforts in addressing this problem have focused on promoting student learning of complex systems. There are also a few studies that examine this problem from a teacher perspective. While these endeavors have yielded various successes and discoveries, the findings concerning teachers' complexity understanding and instructional practices are not conclusive. This is because most studies are small-scale, involve selective teachers, or investigate singular aspects of complex systems understanding. In short, we have yet to gain a thorough insight of the extent science teachers understand and teach complex systems. This research addresses the gaps directly by looking at science teachers' understanding and teaching of complex systems. It examines what they know and teach about complex systems, how their instructional practices may be influenced by their understanding and why the ideas may be difficult to comprehend and teach. This research was conducted with 90 11th and 12th grades science teachers across six Singapore schools. A mixed methods design was used. The findings revealed that while science teachers might appreciate the complex nature of systems, their understanding was not comprehensive: few teachers had prior knowledge of this domain; and certain complex systems ideas appeared better understood than others. It was also found that complex systems ideas were conveyed in science lessons but the extent the ideas were taught was uneven. These ideas were conveyed more often in biology than in chemistry and physics, and certain ideas were more explicitly taught. Teachers with better complex systems understanding were also better able to convey these ideas in their lessons. Several reasons impeding teachers' understanding and teaching of complex systems were also revealed. Implications for research and professional development for science teachers are discussed.