  We initiate the classification of unitary superconformal defects in unitary
superconformal field theories (SCFT) of diverse spacetime dimensions $3\leq d
\leq 6$. Our method explores general constraints from the defect superconformal
symmetry, unitarity, as well as consistency conditions of local bulk-defect
couplings. Such features are common to all superconformal defects regardless of
any Lagrangian description. In particular, modified Ward identities of
conserved currents in the presence of the defect induce a distinguished set of
conformal primary operators on the defect worldvolume, which includes the
universal displacement operator associated with broken translations transverse
to the defect. Consistency with the preserved superconformal symmetry and
unitarity requires that such operators arrange into unitarity multiplets of the
defect superconformal algebra, which in turn leads to nontrivial constraints on
what kinds of defects are admissible in a given SCFT. We carry out the analysis
explicitly for one-dimensional defects, namely superconformal lines, and leave
the study of higher dimensional defects to forthcoming work. We also comment on
the implications of our results for the deformations of superconformal lines
and one-form symmetries in SCFTs.
