Phages are the most abundant organisms in the biosphere and are a common feature of prokaryotic life. They infect bacteria and archaea, playing key roles in molecular biology, horizontal gene transfer, bacterial evolution, and as diagnostic and therapeutic agents. Understanding phage biology and their interactions with hosts is crucial for understanding microbial systems. This article reviews the roles of phages in different host systems, highlighting the use of modeling, microscopy, isolation, and genomic/metagenomic approaches to study phages. Phages have a significant impact on microbial ecosystems, influencing population dynamics and evolution. The article presents three case studies: marine cyanophages, animal phages, and archaeal viruses. Marine cyanophages are widespread, with significant ecological roles in carbon cycling. Animal phages are abundant in the gut and other body sites, influencing bacterial populations and potentially contributing to disease. Archaeal viruses are less studied but may have important roles in microbial ecosystems. Phage life cycles include lytic, lysogenic, and pseudolysogenic, each with different impacts on host populations. Phage abundance and diversity are studied using various methods, including metagenomics, which allows for the identification of phage genes and their roles in environmental processes. Phage research has a complex history, with early studies focusing on model phages and recent advances in metagenomics providing new insights. Phages play a critical role in shaping microbial communities and their environments, and further research is needed to fully understand their impact.Phages are the most abundant organisms in the biosphere and are a common feature of prokaryotic life. They infect bacteria and archaea, playing key roles in molecular biology, horizontal gene transfer, bacterial evolution, and as diagnostic and therapeutic agents. Understanding phage biology and their interactions with hosts is crucial for understanding microbial systems. This article reviews the roles of phages in different host systems, highlighting the use of modeling, microscopy, isolation, and genomic/metagenomic approaches to study phages. Phages have a significant impact on microbial ecosystems, influencing population dynamics and evolution. The article presents three case studies: marine cyanophages, animal phages, and archaeal viruses. Marine cyanophages are widespread, with significant ecological roles in carbon cycling. Animal phages are abundant in the gut and other body sites, influencing bacterial populations and potentially contributing to disease. Archaeal viruses are less studied but may have important roles in microbial ecosystems. Phage life cycles include lytic, lysogenic, and pseudolysogenic, each with different impacts on host populations. Phage abundance and diversity are studied using various methods, including metagenomics, which allows for the identification of phage genes and their roles in environmental processes. Phage research has a complex history, with early studies focusing on model phages and recent advances in metagenomics providing new insights. Phages play a critical role in shaping microbial communities and their environments, and further research is needed to fully understand their impact.