Human adult spermatogenesis involves a balance of spermatogonial stem cell self renewal and differentiation, alongside complex germline-niche interactions. To better understand, we performed single cell RNA sequencing of ~7000 testis cells from three healthy men of peak reproductive age. Our analyses revealed multiple distinctive transcriptional 'states' of self-renewing and differentiating spermatogonia, the cellular stages of gametogenesis, five niche cells (Leydig, Myoid, Sertoli, Endothelial, macrophage) and insights into germline-niche communication. Spermatogenesis was reconstructed computationally, which identified sequential coding, noncoding, and repeat-element transcriptional signatures. A new, developmentally early and likely quiescent spermatogonial state is identified (GFRA1-/ETV5-/ID4+/UTF1+/FGFR3+). Notably, certain epigenetic features combined with nascent transcription analyses suggest considerable plasticity within certain spermatogonial populations/states. Key findings were validated via RNA and protein staining. Taken together, we provided the first “Cell Atlas” of the adult human testis, and provide multiple new insights into germ cell development and germ cell – niche interaction. Overall design: We isolated single testicular cell from two infant (13 months old). Two technical replicates were performed for each individual.