© 2020 Elsevier Inc. This chapter introduces the polarized neutron reflectometry (PNR) technique with a focus on its unique applications to studying the effects of light elements and ion beams in magnetic thin films. The chapter is divided into six sections. Following a brief introduction in Section 1, Section 2 introduces the operational principles and advantages of PNR. Section 3 discusses recent experiments on magnetic hydrogen sensors using in-situ magnetic measurements made on a PNR beam line. Section 4 reviews recent progress using PNR to clarify how low-energy ion beams can modulate the magnetic properties by implantation, modifying oxygen stoichiometry, interface engineering with argon, and imprinting magnetic domains by driving phase transitions. Section 5 exemplifies how PNR can be used to study lateral magnetic domain structures patterned using helium ion beams. Section 6 presents conclusions and future perspectives in form of a brief roadmap highlighting some of the latest developments in PNR, and the new technical possibilities that are anticipated over the coming decade.