This chapter focuses on the use of Extreme-Mass-Ratio Inspirals (EMRIs) to test General Relativity (GR) and alternative theories of gravity. EMRIs are binary systems where one object is much more massive than the other, leading to complex gravitational wave (GW) dynamics. The chapter discusses the astrophysical mechanisms that produce EMRIs, their detectability, and the modeling of their evolution. It highlights the importance of environmental effects, such as accretion structures and dark matter distributions, and the impact of spin on the motion. The chapter also explores tests of black hole properties, including multipole moments, tidal effects, and horizon absence, as well as tests beyond scales and the gravitational spectrum, such as polarization components and propagation of gravity. The potential of EMRIs for fundamental physics, including the existence of new fundamental fields, is emphasized. The chapter concludes with an outlook on the future of EMRI observations and the role of space-based detectors like LISA in advancing our understanding of gravity and astrophysics.This chapter focuses on the use of Extreme-Mass-Ratio Inspirals (EMRIs) to test General Relativity (GR) and alternative theories of gravity. EMRIs are binary systems where one object is much more massive than the other, leading to complex gravitational wave (GW) dynamics. The chapter discusses the astrophysical mechanisms that produce EMRIs, their detectability, and the modeling of their evolution. It highlights the importance of environmental effects, such as accretion structures and dark matter distributions, and the impact of spin on the motion. The chapter also explores tests of black hole properties, including multipole moments, tidal effects, and horizon absence, as well as tests beyond scales and the gravitational spectrum, such as polarization components and propagation of gravity. The potential of EMRIs for fundamental physics, including the existence of new fundamental fields, is emphasized. The chapter concludes with an outlook on the future of EMRI observations and the role of space-based detectors like LISA in advancing our understanding of gravity and astrophysics.