This paper sought to explore the Knowledge of Algebra for Teaching and the algebra difficulties of In-field and Out-of-field Junior High School mathematics teachers using the expanded KAT framework. The study employed the descriptive survey design and involved the participation of 374 mathematics teachers using an achievement test instrument. The study projected School Algebra Knowledge as the prevailing knowledge domain and revealed that the two categories of Junior High School mathematics teachers possess knowledge that falls below average for five (5) out of the seven (7) algebra knowledge domains and also for the overall Knowledge of Algebra for Teaching. However, In-field mathematics teachers showed higher knowledge as compared to Out-of-field mathematics teachers in six (6) out of the seven (7) algebra knowledge domains as well as the overall Knowledge of Algebra for Teaching. Also, the research revealed that mathematics teachers with 5 or more years of teaching experience have higher Knowledge of Algebra for Teaching as compared to those with below 5 years of teaching experience. The study finally identified eight major algebra difficulties among Junior High School mathematics teachers. The findings of the study have implications for teacher preparation, policy and practice.