From 2016 to 2021, the Better Math Teaching Network (BMTN) aimed to transform high school mathematics teaching in New England. Researchers and teachers worked together to make high school Algebra I classes more student centered. Network leaders organized the BMTN as a networked improvement community (NIC) to address a common problem of practice using improvement science. The BMTN was piloted with a group of nine teachers during the 2015-2016 school year and added teachers the following three years. In all, a total of 63 teachers engaged in the BMTN. Selected from a pool of volunteers that applied to join the network, participating teachers worked in urban, suburban, and rural contexts and taught at least one Algebra I course to 9th grade students. They engaged collaboratively to continuously improve their teaching, enhancing learning for thousands of high school math students throughout New England. This report is a developmental evaluation that studied and supported the networked improvement community's (NIC) initiation, development, outcomes, and dissemination of lessons learned. The developmental evaluation of BMTN aimed to: (1) infuse an evidence-based critical friend/thought partner perspective into the network development process; (2) track growth and the development of the NIC as a learning organization; (3) produce useable knowledge for the education field and specifically for other educators, policymakers, funders, and researchers interested in the NIC model as a way to organize for improvement and address high-leverage practical problems; and (4) advance the evaluation field by testing and refining models for evaluating improvement processes and NICs in education contexts.