Transcriptional regulatory networks take part in the regulation of essentially every aspect of an organism's biology. Yet, the structure of these networks can be baroque and attempts to rationalize these structures based on network engineering principles have met with mixed success. In my graduate work, I have examined how the structure of modern transcriptional regulatory networks is rooted in their evolutionary history. In this light, an understanding of transcriptional regulatory network structure based exclusively on engineering design principles may not be useful. Instead, to rationalize the structure of transcriptional regulatory networks requires the careful examination of how these structures are built using pieces of existing networks, how networks are rewired by the constant force of degenerative mutation, and how constraints on transcriptional regulatory network evolution shape the path towards the gain of new functions.