03/22/2012 11:01 am ET Updated May 22, 2012

18 Awesome Pieces Of Street Art (PHOTOS)

I try to stay as invisible as possible when I'm on the street taking pictures. This is where my ancient CIA training comes in handy: I'm a guy with a camera who can easily blend in. I've never heard the words, "Don't take a picture of that," because most people don't even notice that I'm taking a picture.

Over the last four decades, the definition of what is or isn't street art has been debated by dealers, academics, and artists themselves; it seems to defy clear categorization. I'm not a dealer, an academic, or a street artist, and my own definition has remained constant: Street art--and the street art I photograph--must be in a public place, must be visually captivating and perhaps beautiful, must be the product of an informed intellect (not of a vandal), and must be purposeful. It can be a jolt by a single artist or a chaotic improvisation by dozens of different hands in unexpected places, in unexpected ways.

The street art I choose to photograph also calls to mind Duke Ellington's take on music: "If it sounds good, it is good." It's all personal. To me, if it looks good, it is good and I want to preserve it in a photograph. And if it's bad, which usually means gratuitous vandalism--I'll keep walking. I apply the same standard to my own photographs. They speak for themselves; the viewer will be stimulated or not.

In an episode from Federico Fellini's Roma (1972), workers digging a subway tunnel under the streets of Rome come upon what appears to be an ancient ruin. Archaeologists are called to document the find, but when they enter the series of long-forgotten rooms and shine their lamps on the frescoes and sculptures, the artworks begin to fade and disintegrate, disappearing before there is a chance to document them.

The art I've found on the walls along the streets of New York City and elsewhere doesn't usually vanish so quickly or dramatically, but it does eventually disappear, often without a trace. Capturing that special moment when it does appear is what my mission is all about.

This is why I've taken thousands of photographs of street art and why I plan to take thousands more. And with every picture, I always hope I'm doing right by Walker Evans, Berenice Abbott, ALI, and the many other artistsoften unknown who have inspired me to hit the streets practically every day since 1975.

Hank O'Neal is the author of XCIA's Street Art Project: The First Four Decades [SIMAN MEDIA WORKS, $39.95]