In this talk, I explore the set of line defects supported by 3D N=4 theories, and their significance in the context of generalised symmetries. I begin by discussing mirror symmetry of line defects using the example of Sp(k) SQCD and its two mirror theories. I then introduce rational Q-systems, a powerful technique borrowed from spin-chains/integrability, for evaluating twisted indices and studying line operator correlation functions. Finally, I highlight the role of line defects in realising mirror symmetry in the presence of non-trivial higher symmetries