The hierarchical model of galaxy formation predicts that the Milky Way halo is populated by tidal debris from dwarf galaxies and globular clusters. The Gaia mission, which provides extensive data on the positions, motions, and photometry of billions of stars, has revolutionized our understanding of stellar streams in the Milky Way. Gaia has discovered over 100 stellar streams, increased the resolution of stream structures, and constrained their kinematics, providing valuable insights into the Milky Way's dark matter halo. These discoveries have established stellar streams as sensitive gravitational tracers, and future studies aim to map the full population of stellar streams and develop numerical models to capture their evolution. The comparison between these models and observations will help reveal dark-matter subhalos below the threshold for galaxy formation, testing the cold dark matter paradigm on small scales. The review highlights the theoretical and historical context, the methods used for stream discovery and characterization, and the key opportunities and research directions in the Gaia era.The hierarchical model of galaxy formation predicts that the Milky Way halo is populated by tidal debris from dwarf galaxies and globular clusters. The Gaia mission, which provides extensive data on the positions, motions, and photometry of billions of stars, has revolutionized our understanding of stellar streams in the Milky Way. Gaia has discovered over 100 stellar streams, increased the resolution of stream structures, and constrained their kinematics, providing valuable insights into the Milky Way's dark matter halo. These discoveries have established stellar streams as sensitive gravitational tracers, and future studies aim to map the full population of stellar streams and develop numerical models to capture their evolution. The comparison between these models and observations will help reveal dark-matter subhalos below the threshold for galaxy formation, testing the cold dark matter paradigm on small scales. The review highlights the theoretical and historical context, the methods used for stream discovery and characterization, and the key opportunities and research directions in the Gaia era.