Defects and interfaces can have a profound effect on the macroscale physical and chemical properties of nanostructures through modifying their local atomic and electronic structure. While defects and interfaces have been a well-studied subject for decades, little is known about their local atomic and chemical structure, sub-Angstrom structural distortions within their vicinity, or their stability and transition dynamics under extreme conditions. For the past decade, there has been incredible progress in the ability to image and manipulate the atomic and chemical structure of nanomaterials with the development of aberration-corrected scanning/transmission electron microscopy (S/TEM). Using ultra-high resolution aberration-corrected S/TEM imaging and spectroscopy, this talk will discuss our recent efforts on the determination of the atomic and chemical structure of nanomaterials and the sub-Angstrom structural distortions and relaxation effects that occurs around the defects, dopants, domain walls, and interfaces in the family of 2D crystals and complex oxides. In the family of 2D crystal transition metal dichalcogenides (TMDs) alloys, we show how the formation of chemically ordered states and vacancy/dopant coupling leads to unusual relaxation effects around dopant-vacancy complexes. In addition, we explore stability and transition dynamics of defects leading to grain boundary migration in monolayer TMDs under electron beam irradiation. Finally, in the family of complex oxides, we present how ferroelectric polarization emerges at the atomic level across the domain walls and interfaces in single phase and hybrid complex oxide systems and the impact of this emergence on the macroscale properties.