* Early Years (A Fever You Can't Sweat Out era): Often a fairly straight, emo-inspired style with a side fringe, and often dyed black.
* Pretty. Odd. era: This era saw him with a more natural, slightly longer hairstyle, sometimes wavy.
* Vices & Virtues era: A slicked-back, slightly more polished look, often with some volume at the front.
* Too Weird to Live, Too Rare to Die! era: A quiff, often styled with a lot of volume and product.
* Death of a Bachelor era and onwards: He has gone back and forth with a slicked back pompadour and short and messy on top.
In general, Brendon Urie often has a clean cut look.
If you have a specific era in mind, let me know, and I can describe the hairstyle in more detail!