Defects are key to enhance or deploy particular materials properties. In this thesis I present analyses of the impact of defects on the electronic structure of materials using combined experimental and theoretical Electron energy loss spectroscopy (EELS) in (Scanning) Transmission Electron Microscopy. The energy loss near-edge structure (ELNES) in EELS reflects the element-specific electronic structure providing insights into bonding characteristics of individual atomic species. New electron optical devices have boosted the analytical capabilities by which materials can be investigated with atomic resolution and single atom sensitivity using (scanning) transmission electron spectroscopy (STEM/TEM).With the help of aberration correctors for forming small electron probes, high intensity electron beams can nowadays be focused to clearly less than 100 pm which has enhanced the resolution and sensitivity in analytical scanning transmission electron microscopy (STEM). Various kinds of defects in different complex oxides were studied: point defects like oxygen vacancies in BiVO4 and SrMnO3, edge-dislocations in BiFeO3, and planar defects in GaAs. By comparison with experimental data, structures for these systems were proposed based on all-electron density functional theory (DFT) code Wien2k. By comparing theoretical calculations and experimental data, a pronounced surface reduction in the oxidation state of vanadium in BiVO4 from +5 to +4 was unveiled, which is due to a high density of oxygen vacancies, and its importance in potential application of BiVO4 in photoelectrochemical energy conversion. A similar study was performed on a series of SrMnO3 thin films with different epitaxial strain where theoretical investigations revealed the impact of oxygen non-stoichiometry and strain on the O-K ELNES. In the next study, molecular dynamics simulations were combined with FEFF-based EELS calculations and its comparison with experiments was helpful for the correct prediction of the edge dislocation core structure in BiFeO3. This study also confirmed the presence of Fe atoms in the core of the edge dislocation which possibly makes these defects ferromagnetic whereas the bulk structure is known to be antiferromagnetic. This thesis has established methodologies for utilizing different codes, illustrating how links between experimental and theoretical ELNES can be used in revealing structural information around defects and how defects affect materials properties. This tandem methodology of theory and experiments is applicable to various future materials where the reliable interpretation of EELS data is pivotal in unfolding mysteries of such technologically important materials.