  Electronic resonances are states that are unstable towards loss of electrons.
They play critical roles in high-energy environments across chemistry, physics,
and biology but are also relevant to processes under ambient conditions that
involve unbound electrons. This feature article focuses on complex-variable
techniques such as complex scaling and complex absorbing potentials that afford
a treatment of electronic resonances in terms of discrete square-integrable
eigenstates of non-Hermitian Hamiltonians with complex energy. Fundamental
aspects of these techniques as well their integration into molecular
electronic-structure theory are discussed and an overview of some recent
developments is given: analytic gradient theory for electronic resonances, the
application of rank-reduction techniques and quantum embedding to them, as well
as approaches for evaluating partial decay widths.
