This dissertation explores the molecular beam epitaxial (MBE) growth of ferrite-basedferroelectric and multiferroic thin films, with a focus on achieving ultrathin improper ferroelectricity and room-temperature multiferroicity. There has been a long-standing debate regarding how thin an improper ferroelectric filmcan be while still retaining its spontaneous polarization. Theoretical predictions suggest that improper ferroelectricity should persist down to a single formula unit (actually two, due to theoretical constraints), whereas experimental studies have reported that polarization vanishes below six formula units. The first part of this dissertation aims to experimentally validate the theoretical prediction. To achieve this, I designed a dedicated epitaxial template—comprising a bottom electrode and a monolayer bridging layer—and carried out the film growth with atomic-layer precision. This effort culminated in the stabilization of robust and switchable out-of-plane polarization in a 0.75-unit-cell-thick h-LuFeO₃ film, the thinnest improper ferroelectric reported to date. This work not only confirms the theoretical prediction but also highlights the critical importance of structural compatibility—beyond simple lattice match—when interfacing distinct crystal structures. Next, my thesis covers the efforts in developing a new room-temperature relaxormultiferroic material. Multiferroic materials, which simultaneously exhibit two or more ferroic orders, such as ferroelectricity and magnetism, are considered ideal candidates for energy-efficient memory and logic applications. Despite decades of research, multiferroics with both ferroelectric and magnetic Curie temperatures above room temperature are exceedingly rare. To explore pathways toward new room-temperature multiferroics, the second part of this dissertation focuses on engineering the electrical properties of barium hexaferrite (BaFe₁₂O₁₉), a robust room-temperature ferrimagnet. By leveraging a novel substrate and applying epitaxial strain, I demonstrate that BaFe₁₂O₁₉ can be driven into a polar state characterized by the emergence of polar nanoregions. This work establishes BaFe12O19 as a compelling platform for exploring strain-driven multiferroicity and highlights the potential of epitaxial strain engineering to realize new room-temperature multiferroics. The final part of this dissertation focuses on the growth optimization of a family of Rtypeferrites, (Ba,Sr)(Co,Fe)2±xRu4∓xO11. These materials were developed not only as structurally and chemically compatible bottom electrodes for ultrathin improper ferroelectric films in CHAPTER 1, but also show promise for future spintronic and electrocatalytic applications due to their intriguing electronic, magnetic, and chemical properties. Lastly, the growth of Fe57 enriched metallic and oxide films enables and motivates theuse of conversion electron Mössbauer spectroscopy (CEMS) to probe many complex oxide epitaxial heterostructures including superlattices.