Bacteriophages are the most plentiful organisms on the earth and they are a ubiquitous trait of prokaryotic existence. They exist in all environments as a part of a complex microbial ecosystem. A phage is a virus that can infect bacteria where the distribution and abundance are based on the host organisms. Bacteriophages can impact their host populations in different ways and also have impacts on the microbial ecology and geochemical cycles. The concept of phage therapy as a fundamental application of bacteriophages is deeply relying on the facts of their host specificity, abundance in the environment with limited ecological impact. This review focuses on the ecological role of bacteriophages in the marine environment and some applications.