Horizontal gene transfer (HGT) is the acquisition of genetic material from another organism, common in prokaryotes through mechanisms like conjugation, transduction, and transformation. While less frequent in eukaryotes, recent studies using next-generation sequencing reveal its occurrence among fungi, plants, and animals. In plants, HGT has led to the evolution of photosynthetic eukaryotes, exemplified by the transfer of tumor-inducing genes from Agrobacterium to plants. HGT also impacts host range, as seen in the transfer of the ToxA gene enabling Pyrenophora tritici-repentis to cause wheat tan spot. Additionally, HGT can transmit resistance, such as the Fhb7 gene from an endophytic fungus conferring broad resistance to Fusarium species in Thinopyrum elongatum. Understanding HGT's role in eukaryotes remains a focus for future research.