Large since the mid-1800s, the parade has celebrated Irish culture and Irish immigrants, who once faced nativist calls for their exclusion from the workforce - and from the country - when they began arriving in the city in huge numbers during the Irish Famine.
To give you a taste of what the atmosphere is like, here's live video footage of the St. Patrick's Day parade in South Boston.
"The Irish step dancers are Theodora's favorite because she used to be in Irish step dancing classes".
All of the proceeds from the parade will be donated to Birth Haven, a non-profit organization that provides shelter, support and education to pregnant homeless women.
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The parade's organizers were once involved in annual fights over whether to exclude openly gay groups from the march.
Nancy Harrington and her husband, Michael, of Hardyston, were enjoying Saturday's festivities - they also recalled attending the parade since the beginning - waving to local fire and emergency personnel as they slowly passed by with their lights on and their horns blaring.
Parade goer Coralie Draper said, "I love this parade".
The St. Patrick's Day parade takes place during one of the busiest weekends in Pittsburgh.
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Sheila Murphy Crawford was this year's Grand Marshal.
"He's basically the Irish version of Moses and no other Irishman has had an effect on Irish identity than Saint Patrick".
"We come to them all the time".
A Spider-Man actor pretends to shoot webs at the bystanders during the St. Patrick's Day parade on March 17, 2018.
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But perhaps the biggest Irish spirit belonged to Paul Alonyisus McCormick of Chesapeake City, who was decked out in a leprechaun costume.