AIM UP, the NSF REU site at the University of Wisconsin-Milwaukee, will host 12 undergraduates for 8 weeks each summer for the next three years. The project directors will recruit 4 freshmen or sophomores from two-year or four-year colleges and 8 juniors or rising seniors from a range of institutions, including those with limited research opportunities. Research projects spanning pure and applied mathematics, biostatistics, data science, and AI will be guided by experienced faculty and doctoral student mentors. AIM UP aims to increase undergraduate research opportunities in mathematical sciences and to train faculty and future faculty in effective mentoring. The program will enhance students' research skills, advance the science of each REU project, and demonstrate the impact of mentor training and program activities on undergraduate research and mentoring. AIM UP actualizes a four-stage pipeline involving early career undergraduates, later career undergraduates, graduate student mentors, and faculty mentors. Participants will engage in professional development tailored to their stage, including workshops on mentoring, collaboration, communication, and conflict resolution. In a vertically layered environment, scholars will gain research experience in areas like abstract algebra, combinatorics, topology, biostatistics, data science, AI, or probability, as well as professional development skills including research dissemination and career preparation. Each research team is