Today is Ash Wednesday.
For many of us, the smudge on people's foreheads signifies the first day of Lent.
Photographer Greg Miller has been documenting this ritual on the streets of New York City for the past 20 years.
His upcoming book, Unto Dust, features 40 portraits from his decades of work.
Here is what he told us about the project.
Why did you decide to do this project?