Heterostructure thin films combine dissimilar materials in stacked architectures to engineer interface-driven properties that are unattainable in single layers. This chapter reviews the fundamentals that control functionality in such systems, including band alignment, depletion and built-in fields, and strain engineering. We outline growth fundamentals and survey major fabrication routes, physical and chemical vapor deposition, pulsed laser deposition, atomic layer deposition, and molecular beam epitaxy, highlighting their strengths and limitations for layer control, uniformity, and interface sharpness. Advanced characterization methods spanning structural (XRD, TEM/STEM, AFM), compositional (EDS, EELS, ToF–SIMS, PEEM/SPLEEM), electrical (Hall, four-probe, van der Pauw, C-V/impedance), and optical analyses are summarized, with emphasis on interface and defect assessment (CDI, XANES/EXAFS, XPS). We then connect multifunctional mechanisms, ferroelectricity and magnetoelectric coupling, thermoelectric and piezoelectric conversion, and electro-optic/photovoltaic responses, to device implementations including sensors and actuators, spintronic elements, non-volatile memories (RRAM, FeRAM/FeFET), photodetectors, solar cells, and flexible/wearable electronics. Recent case studies (e.g., LaAlO3/SrTiO3 2DEGs, multiferroic and van der Waals stacks, hybrid organic–inorganic systems) illustrate interface-engineered performance. We close with key challenges in scalability, interface stability, and CMOS integration, and outline opportunities in AI-assisted process optimization and quantum/topological heterostructures for next-generation, energy-efficient electronics.