Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Deviation / Discrepancy - Procedural Clearance; Ground Incursion Taxiway
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented
Airport; Human Factors
We were taxiing from the FBO to Runway 30 at IAD.  The airport was not very busy but we had been given several taxi and hold short clearances along the lengthy taxi.  We were southbound on Zulu taxiway and were to hold short of Echo.  We were actively looking for Echo and I had asked my partner where Echo was; his first explanation was erroneous and he quickly corrected himself but with a vague description.  Or at least my understanding was vague.  I had asked for clarification so he was looking down at the chart for the exact location again and during this time we had taxied into the intersection of Z & E.  We could then see the signs but it was too late; we were there.  About that time ground spoke up and stated the obvious and scolded us for the mistake.  He said it was ok and don't worry about it.  While we were passing through Echo we looked closely and noticed a lack of taxiway markings for Echo.  I think local ground controllers take for granted that some operators go that route a lot and are familiar with signage.  We were slightly mentally tired however we were vigilant but just missed it.  I really don't feel fatigue played too much of a role in this event.  We were looking around and couldn't find the signs for Echo until it was too late.  It would have been helpful to have a chart in front of me to read.  My captain was just as confused as I was about where Echo was.  With only a few seconds to digest the info and find it on the chart then try to locate it on the airport was definitely a challenge.  One that we failed.  My question for this is why can't Dulles improve their taxi signage?  It seems other large airports do better.  Especially where there are very large taxi apron areas.  It can be difficult to distinguish which specific taxi lanes are meant for which signs.
Citation First Officer as the pilot taxing the aircraft; reports a taxiway incursion at IAD due to lack of painted taxiway markings.