Although the genes essential for life have been identified in less complex model organisms, their elucidation in human cells has been hindered by technical barriers. We use extensive mutagenesis in haploid human cells to identify approximately 2,000 genes required for optimal fitness under culture conditions. To study the principles of genetic interactions in human cells we created a synthetic lethality network focused on the secretory pathway based exclusively on mutations. This revealed a genetic crosstalk governing Golgi homeostasis, an additional subunit of the human oligosaccharyltransferase complex, and a Phosphatidylinositol 4-Kinase Beta adaptor hijacked by viruses. The synthetic lethality map parallels observations made in yeast and projects a route forward to reveal genetic networks in diverse aspects of human cell biology.