Summary: Many students will need remedial work in mathematics during their high school years. Some sort of help will be needed to fulfill the National Council of Teachers of Mathematics’ (NCTM’s) vision of a mathematics classroom that involves students having access to mathematically rich problems and being engaged in solving them. The high school where these authors taught is located in a large urban school district in Kentucky. The school’s enrollment is 958 students, of whom 70 percent qualify for free or reduced-cost lunches. For the past three years, the authors have been involved in the development of an after-school remediation and enrichment program for high school students that they call the Mathematics Lab. The conception for the Mathematics Lab ‒ five one-and-a-half hour after-school sessions that run from October through March ‒ grew out of an idea proposed by the school’s mathematics resource teacher. It has since become a collaborative program; teachers work together to design and implement the program and to recruit students. The program’s goals are to intervene with students who are struggling to pass state mathematics assessments and to build a community of students excited about mathematics. For the program to be successful, students must be excited about participating in the labs and spending time learning after school. This article will describe the Mathematics Lab’s structure, the implementation process, and how student achievement data are used to inform development. (ERIC)