Modern materials physics and nanoscience aim to control materials and their interfaces at the atomic level. However, interfaces between polar and non-polar layers face a "polar catastrophe," which forces interfacial reconstruction. In traditional semiconductors, this is achieved through atomic disordering and stoichiometry changes, but in multivalent oxides, electrons can move, allowing atoms to remain in place. Atomic-scale electron energy loss spectroscopy reveals an asymmetry between ionically and electronically compensated interfaces, affecting sharpness and carrier density. This suggests a strategy to design sharp interfaces, remove interfacial screening charges, and control band offsets, improving oxide device performance. Oxide thin films have diverse industrial applications, from electronics to high-frequency filters. Their wide range of ground states offers richer functionality than conventional semiconductors, such as piezoelectric resonators and magneto-optical storage. Atomic-layer control of growth enables coupling of different physical properties at the microscopic level. Interface effects dominate in some cases, introducing novel considerations at small length scales. Electrostatic boundary conditions significantly influence atomic and electronic structures at solid-solid interfaces. Even neutral planes can have interface dipoles due to band offsets and bond polarizations. For ionic materials, polar discontinuities increase energy costs for abrupt heterointerfaces, leading to catastrophic roughening during growth unless composition is graded at the interface to avoid net formal charge. The response to this energy cost affects electrical and physical properties, such as interface phases or roughness. A simple electrostatic model explains these behaviors, tested experimentally for (001) interfaces between SrTiO3 and LaAlO3. The (001) planes in ABO3 perovskite structures alternate between AO and BO2 layers. Different valence states allow neutral structures. In A2+B4+O3, AO and BO2 planes are neutral. In A3+B3+O3, AO and BO2 planes have opposite charges. Joining perovskites from different charge families with atomic abruptness creates polar discontinuities. For LaAlO3 and SrTiO3, two configurations arise. The AlO2/LaO/TiO2 interface (n-type) has excess electrons, while AlO2/SrO/TiO2 (p-type) has excess holes. The n-type interface is compensated by mixed-valence Ti states, while the p-type is compensated by oxygen vacancies. Experimental evidence shows that the n-type interface has excess electrons, while the p-type has oxygen vacancies. The n-type interface is rougher than the p-type. The presence of oxygen vacancies at the n-type interface suggests a mechanism to reduce band offset without divergence. Adding vacancies and compensating electrons maintains net charge, introducing an interface dipole that shifts the band offset. Controlling interface termination layers allows tuning between insulator and conductor, trading chemical for electrical roughness. The band offset can beModern materials physics and nanoscience aim to control materials and their interfaces at the atomic level. However, interfaces between polar and non-polar layers face a "polar catastrophe," which forces interfacial reconstruction. In traditional semiconductors, this is achieved through atomic disordering and stoichiometry changes, but in multivalent oxides, electrons can move, allowing atoms to remain in place. Atomic-scale electron energy loss spectroscopy reveals an asymmetry between ionically and electronically compensated interfaces, affecting sharpness and carrier density. This suggests a strategy to design sharp interfaces, remove interfacial screening charges, and control band offsets, improving oxide device performance. Oxide thin films have diverse industrial applications, from electronics to high-frequency filters. Their wide range of ground states offers richer functionality than conventional semiconductors, such as piezoelectric resonators and magneto-optical storage. Atomic-layer control of growth enables coupling of different physical properties at the microscopic level. Interface effects dominate in some cases, introducing novel considerations at small length scales. Electrostatic boundary conditions significantly influence atomic and electronic structures at solid-solid interfaces. Even neutral planes can have interface dipoles due to band offsets and bond polarizations. For ionic materials, polar discontinuities increase energy costs for abrupt heterointerfaces, leading to catastrophic roughening during growth unless composition is graded at the interface to avoid net formal charge. The response to this energy cost affects electrical and physical properties, such as interface phases or roughness. A simple electrostatic model explains these behaviors, tested experimentally for (001) interfaces between SrTiO3 and LaAlO3. The (001) planes in ABO3 perovskite structures alternate between AO and BO2 layers. Different valence states allow neutral structures. In A2+B4+O3, AO and BO2 planes are neutral. In A3+B3+O3, AO and BO2 planes have opposite charges. Joining perovskites from different charge families with atomic abruptness creates polar discontinuities. For LaAlO3 and SrTiO3, two configurations arise. The AlO2/LaO/TiO2 interface (n-type) has excess electrons, while AlO2/SrO/TiO2 (p-type) has excess holes. The n-type interface is compensated by mixed-valence Ti states, while the p-type is compensated by oxygen vacancies. Experimental evidence shows that the n-type interface has excess electrons, while the p-type has oxygen vacancies. The n-type interface is rougher than the p-type. The presence of oxygen vacancies at the n-type interface suggests a mechanism to reduce band offset without divergence. Adding vacancies and compensating electrons maintains net charge, introducing an interface dipole that shifts the band offset. Controlling interface termination layers allows tuning between insulator and conductor, trading chemical for electrical roughness. The band offset can be