The significance of polycrystalline ceramic oxides as material candidates for applications in and beyond electronics, information technology and energy storage/conversion is undeniable. However, to further enhance the functionality and scalability of polycrystalline oxides for real-world applications, considerable efforts have been focused on tuning macroscopic properties including electrical conductivity through the design of new chemistries and structures at atomic scale. Of particular interest, is understanding the role of grain boundary (GB) chemical segregation in electrical conductivity, to engineer effective GBs with improved electrical conductivity. GBs make up a considerable volume fraction of polycrystals, particularly in nano sized grains and have different structure and chemistry and therefore properties compared to that of the grains. While the influence of GB segregation in electrical conductivity of most binary oxides (containing one cation) and slightly doped, dilute solid solutions (containing < 1 mol% dopant concentration) is extensively studied, there is a noticeable gap in understanding GB segregation behavior in complex oxides, which comprise the most technologically important ceramics. In this work, we thus seek to expand the understanding of GB segregation in 2 different complex oxides defined here as oxides containing 2 or more cation species and more than 1 mol % of each in the lattice, with the goal of improving GB and total electrical conductivity. The first part of this work aims to simulate cross-GB oxygen ion (O2-) conductivity in a concentrated gadolinium-doped ceria (Gd0.25Ce0.75O2-) model system using the experimentally measured defect distribution profiles near the GB. Using aberration-corrected STEM-EELS and ELNES, we quantify Gd segregation, oxygen vacancy ("V" _"O" ^"••" ) depletion and Ce3+ segregation to 5 random GBs. This information is then used to develop a phase field model that calculates defect-defect interaction energies (that were ignored in conventional models for dilute solid solutions), and inputs them to the Martin-Nakayama model, to predict the conductivity across each individual GB. Our results suggest that unlike in dilute solid solutions, where conductivity has a monotonic relationship with the concentrations of "V" _"O" ^"••" near the GB, conductivity in a concentrated gadolinium-doped ceria is not solely dependent on the "V" _"O" ^"••" concentrations. Instead, conductivity tends to peak when optimal amounts of "V" _"O" ^"••" are present. This is a reasonable observation, considering that the interactions between defects (e.g., "V" _"O " ^"••" and Gd solutes) influence the mobility and migration of O2-across GBs. In summary, we divide the studied GBs into 2 categories: “type a” and “type b”. Unlike the type a GB, which is likely a better O2-conductor with higher "V" _"O" ^"••" depletions (~ 0.14 – 0.15 mole fraction which is the optimal amount mentioned above), type b GB is likely a poor O2- conductor with lower "V" _"O" ^"••" depletions (~ 0.7 - 0.9 mole fractions). It should be noted that both type a and type b GBs have similar amounts of Gd3+ dopant and Ce3+ local electron concentrations. The second part of this work explores the changes in the GB chemistry of a single phase (CoCuNiZnMg)O entropy stabilized oxide (ESO) rocksalt as it undergoes heat-treatment induced phase transformation. We hypothesized that the phase transformations result in changes in the concentrations of charge carrying defects in grains and at the GBs, affecting charge transport properties. Aberration-corrected STEM coupled with EDS and EELS is used to study the phase evolution at atomic-scale, by analyzing one single-phase (ESO-single) and two multiphase ESOs (heat treated at for 2 and 24 h at 700 C - ESO-2h and ESO-24h). Our findings directly reveal that Cu segregates to the GBs in ESO-single, with homogenous distribution of all cations to the grains. In multiphase ESOs, an enthalpy stabilized Cu-rich tenorite secondary phase forms at some GBs in ESO-2h and at all GBs in ESO-24h. Additionally, Cu-rich tenorite and Co-rich spinel secondary phase particles are found in grains of ESO-24h. Changes in the electrical conductivity are then measured using electrochemical impedance spectroscopy (EIS). No resistivity contribution is detected for the GBs in ESO-single, suggesting that the insignificance of GB space charge potential. Upon heat treatment, the electrical conductivity of grains increases by 2 and 4 orders of magnitude in ESO-2h and ESO-24h, respectively. We attribute this significant enhancement to the formation of additional Cu+/Cu2+ pairs in grains introduced by the secondary phases. As hypothesized earlier, the altered GB chemistry plays a significant role in dictating charge transport properties of this ESO by allowing the nucleation and formation of secondary phases. We find that, while the conductivity across GBs covered by tenorite particles is much lower than that of the grains, ESO’s total conductivity still experiences a significant increase in this system with ~2-"μm" average grain size.