Strongly correlated nanostructures and lattices ofelectrons are studied when these systems reside in a steady-state nonequilibrium. Much of the work done to date has made use of the nonequilibrium real-time Keldysh Green function technique. These methods include: the Keldysh Green function perturbation theory, time-dependent numerical renormalization group, density matrix renormalization group, and diagrammatic quantum Monte Carlo. In the special case of steady-state nonequilibrium we construct an imaginary-time theory. The motivation to do this is simple: there exist an abundant number of well-established strongly correlated computational solvers for imaginary-time theory and perturbation theory on the imaginary-time contour is much more straightforward than that of the real-time contour. The first model system we focus on is a strongly interacting quantum dot situated between source and drainelectron reservoirs.^The steady-state nonequilibrium boundary condition is established by applying a voltage bias $\Phi$ across the reservoirs, in turn modifying the chemical potentials of the leads. For a symmetric voltage drop we have$\mu_{source}=\Phi/2$ and $\mu_{drain}=-\Phi/2$. The dynamics of the electrons are governed by the Hamiltonian $\hat{\mathcal{H}}$ which is inherently independent of the imbalance in the source and drain chemical potentials. The statistics though are determined by the operator $\hat{\mathcal{H}}-\hat{\mathcal{Y}}$, where $\hat{\mathcal{Y}}$ imposes the nonequilibrium boundary condition. We show that it is possible to construct a single effective Hamiltonian $\hat{\mathcal{K}}$ able to describe both the dynamics and statistics of the system. Upon formulating the theory we explicitly show that it is consistent with the real-time Keldysh theory both formally and through an example using perturbation theory.^In these systems there exists a strong interplay between the interactions and nonequilibrium leading to novel nonperturbative phenomena. Therefore, we combine our theory with the Hirsch-Fye quantum Monte Carlo algorithm to study these effects. We then propose a nonequilibrium Hubbard model in the special limit of infinite dimensions (dynamical mean-field theory) where the problem is reduced to solving a self consistent nonequilibrium impurity model. The final chapter concentrates on electron spin and charge filtering through a quantum dot embedded Aharonov-Bohm interferometer.