The "Photorhabdus Virulence Cassette" (PVC) is an elaborate macromolecular protein complex evolved to specifically and potently deliver effector molecules to the interior of target cells. A PVC can be thought of as a "headless bacteriophage", with a protein cargo rather than a nucleic acid one. This thesis sheds light on some of the 'dark matter' in the content and operon structure of PVCs, though mysteries remain. The Introduction guides the reader through the universe of similar protein complexes. The first 2 results chapters 'pave the way' for lab experiments by examining the PVCs in a number of computational workflows. In Chapter 3, the disconnect between sequence and structure for proteins is examined - and what this means for PVCs. Through sensitive methods, less dependent on sequence, new, informative homologies are detected, and we get a 'first look' at the likely structure for many elements of a PVC. Chapter 4 explores the operon structure of PVCs, identifying proteins which may be non-essential to PVC function, and demonstrating the 'microevolution' in, and variability between, operons. In the first of the 'wet lab' chapters, Chapter 5 examines the enigmatic proposed 'tail fibres' of the PVCs, identified as hypervariable in Chapter 4. This chapter represents the first experimental study of a potentially unique chimeric fibre protein, and provides the first experimental data confirming their true nature as bona fide tail fibres. Finally, Chapter 6 details the efforts made to heterologously clone controllable PVCs - a non-trivial task it transpires; and to understand the regulation and population dynamics of how PVCs are deployed by Photorhabdus naturally, with preliminary observations implicating a role for RfaH-like transcriptional regulation proteins and anti-termination/operon polarity suppression.