The Better Math Teaching Network (BMTN) is a networked improvement community (NIC) of researchers and high school teachers from New England who use improvement science principles to deepen student engagement in algebra content. The BMTN began as a pilot during the 2015-16 school year and concluded in fall 2021. Key learnings from the network include the following: (1) The Plan-Do-Study-Act (PDSA) approach to the testing of instructional routines proved to be a durable structure for teachers to reflect upon and improve their instruction; (2) The tested routines most commonly focused on improving students' mathematical justifications and problem-solving skills; (3) Network-developed rubrics and learning activities helped teachers improve opportunities for their students to deeply engage with math content; (4) Network learning activities enhanced teacher learning and supported the spread of promising ideas within the network; and (5) Over time, teachers and researchers spread learnings to educators outside the network through multiple modes and channels of communication.