In Illinois, a Private Detective is the license holder. A Private Investigator must work for a Private Detective through the Private Detective’s Agency. As a side note, a Private Detective Agency must be a Corporation. The Private Detective holds the State License and the Private Investigator does not. He is only an employee. Many times I have seen where both of those terms are interchanged. Even I use those two terms interchanged on this website. Hum? And now you know. Stay Safe.