Core−shell nanoparticles are functional materials with tailored properties, able to improve the requirements of various applications. Both core and shell components can be inorganic or organic, and there are numerous studies in this field regarding their synthesis methods, properties, and applications. This review aims to study core−shell nanostructures with Fe3O4 cores and different shell types, observing their antibacterial and anticancer properties. By the type of coating, Fe3O4 core−shell nanoparticles (NPs) are classified into four categories: metal-coated NPs, metal-organic framework (MOF) coated NPs, metal oxide coated NPs, and polymer-coated NPs. Each category is briefly presented, emphasizing anticancer or antibacterial properties and specific applications (cancer diagnosis or therapy, drug carrier). Moreover, synthesis methods and particle size for both core and shell nanostructures, as well as the magnetic properties of the final core−shell material, are summarized in this review. Most of the consulted papers discussed sphere-like core−shell nanoparticles obtained by chemical methods such as coprecipitation, hydrothermal, and green synthesis methods using plant extract. These types of core−shell nanoparticles could be used as drug nanocarriers for tumor-targeted drug delivery, hyperthermia treatment, or contrast agents. Further work needs to be conducted to understand nanoparticles’ interaction with living cells and their traceability in the human body.