Bacteriophages are the most common biological entities and found in every biosphere, ranging from the human gastrointestinal tract and petrified mummies to acid-mine drainage. Depending on nucleic acid composition, capsid symmetry, and presence or absence of a membrane or envelope, bacteriophages can be classified into eleven families. Contrary to mammalian viruses, relatively few bacteriophages have an internal or exterior membrane or lipidic layer. How enveloped phages cross the 3-layerd bacterial envelope remains enigmatic. In this study, we elucidated the molecular and structural details of host penetration by membrane containing phages at unprecedented resolution by combining complementary methods, including biochemistry, super-resolution fluorescence microscopy, and electron cryotomography (cryo-ET). We revealed that these phages undergo striking morphological changes during their attachment to the host. We were able to capture the structural changes of enveloped phages during their infection process using cryo-ET and sub-tomogram averaging. Our current work provides mechanistic insights into the entry process of these lipid layer containing phages into their bacterial host.