Rank-metric codes that are linear over an extension field F(qⁿ) admit a geometric description in terms of q-systems, i.e., F(q)-subspaces of a vector space over F(qⁿ). In this dictionary, the weights of the code are encoded in how the q-system meets F(qⁿ)-subspaces, and F(qⁿ)-linear MRD codes whose length exceeds n correspond to maximum h-scattered subspaces. Whether maximum h-scattered subspaces exist for all admissible parameters is a natural open question, which so far has been settled only for some special families of parameters. In this talk I will introduce rank-metric codes, q-systems and h-scattered subspaces, review what is known about this existence problem, and present new existence results over arbitrary finite fields that cover broad ranges of admissible parameters.