Soma-germline interactions play conserved essential roles in regulating cell proliferation, differentiation, patterning, and homeostasis in the gonad. In the Drosophila testis, the JAK-STAT, Hedgehog, BMP and EGF pathways are used to mediate soma-germline communication via paracrine signalling. In this thesis, I aim to shed light on the role of juxtacrine signalling, mediated by gap junction proteins, during early spermatogenesis in the fly testis. My analysis demonstrates that gap junctions also mediate direct, bi-directional signalling between the soma and germline. When gap junctions between the soma and germline are disrupted, germline differentiation is blocked and germline stem cells are not maintained. In the soma, gap junctions are required to regulate proliferation and differentiation. Gap junctions are present between germline and somatic stem cells, as well as between differentiating cells by ultrastructural analysis. Localization and RNAi-mediated knockdown studies reveal that gap junctions in the fly testis are heterotypic channels containing Zpg (Inx4) and Inx2 in the germline and in the soma, respectively. Preliminary structure-function analysis of Zpg reveals that the C-terminus of the protein is essential for its function. Furthermore, gap junctions in the testis may be important for mediating calcium signalling. Overall, my results show that bi-directional gap junction-mediated signalling is essential to coordinate the soma and germline to ensure proper spermatogenesis in Drosophila. Moreover, I show that stem cell maintenance and differentiation in the testis are directed by gap junction-derived cues.