Higher order thinking skill (HOTS) is one of the students' abilities that should be developed through teaching and learning. Teachers' knowledge about HOTS and its teaching and learning tactics is a key to successful education. The purpose of this research is to describe teachers' knowledge about higher order thinking skills (HOTS). The research involves qualitative study with the phenomenological approach. The research participants are 27 mathematics teachers from state and private junior high schools across 7 provinces in Indonesia. The researcher collected data with a test followed by focus group discussion (FGD) and interviews. The analysis of data involved Bogdan & Biklen model and descriptive statistics for data from the test. The analysis of FGD, and test data intends to get information on 6 sub-themes; teachers' knowledge about HOTS, importance of HOTS, teaching about HOTS to students, improving students' HOTS, measuring and assessing HOTS, and teachers' ability for solving HOTS-based problems. The results indicate that teachers' knowledge about HOTS, their ability to improve students' HOTS, solve HOTS-based problems, and measure students' HOTS is still low. There are facts, however, that teachers already understand the importance of HOTS and teaching it by using various innovative learning models.