Extracellular Vesicles (EVs) are small membranous particles secreted by the cells. They play an important role in intercellular communication and can transport a variety of biomolecules, including proteins, lipids, and nucleic acids, to target cells. The scientific community recently considered EVs attractive candidates for developing targeted drug delivery systems (DDSs), given their biocompatibility, low immunogenicity, stability in biofluids, and capability to cross biological barriers. Most studies have shown the feasibility of incorporating desired moieties at the EV surface through the genetic modification of EV-producing cells, exploiting the fusion with proteins enriched at the EV membrane. Tetraspanins are transmembrane proteins enriched in EVs, already exploited for EV isolation or tracking upon fusion with fluorescent reporters. CD81 is a well-characterized tetraspanin with ubiquitous protein expression, overexpression tolerance and a limited number of encoded protein isoforms with respect to other EV-associated tetraspanins. Here, I have explored a CD81-based approach for EV targeting. CD81, in full-length or truncated form, was used to guide the expression into EVs of an anti-HER2 moiety, namely the light chains of trastuzumab, within three different constructs, including turbo-GFP (tGFP) as a reporter: CD81-tGFP as master control, CD81-antiHER2-tGFP and CD81delta-antiHER2-tGFP. The first part of the project was dedicated to the characterization of chimeric proteins at cellular and vesicular levels. CD81-based constructs were successfully expressed in HEK239T cells with a preferential enrichment in organelle fractions, underlying the expected involvement in the intracellular vesicular trafficking. Next, chimeric proteins were also found in the derived EVs, with a similar expression trend, corroborated by imaging flow cytometry. Nanoparticle tracking analysis and cryogenic electron microscopy acquisitions confirmed that CD81-fusion proteins boosted EV release without altering the size distribution. Subsequently, I tested the binding capacity of the chimeric proteins to HER2 receptor through orthogonal techniques, such as AlphaLISA and immunoprecipitation. Confocal imaging, also on live cells, confirmed EV internalization into breast cancer cells, depending on the recipient cell type and the presence of HER2 receptor. Moreover, chimeric EVs loaded with doxorubicin were able to mediate a concentration-dependent cytotoxic effect on recipient breast cancer cells. Of note, messenger RNA provided a valuable readout of the in vivo delivery capability of the CD81-engineered EVs, since detected by digital droplet PCR in breast cancer tumour xenografts from mice treated with chimeric EVs. The results presented in this thesis highlighted the feasibility of using CD81 fusion proteins for cell targeting and cargo delivery, ultimately opening new perspectives for the development of EV-based therapeutics.