Once upon a time, tipping was relatively simple. You might add 10% to 15% to your restaurant bill for the server or hand a dollar or two to the valet who parked your car. But now, it feels as though tipping has become the expectation everywhere.

“Technology is great, but you can also utilize it to push the boundaries,” says Sam Zietz, CEO of GRUBBRR, a company that provides self-ordering solutions such as kiosk software to restaurants.

Today’s sales terminals make it easy to ask for tips as part of a transaction, which is why customers may find themselves facing a tipping screen even in retail locations.Still, Zietz believes people shouldn’t feel pressured to tip every time they’re asked. For example, the clerk at your local bookstore probably doesn’t require a tip for groceries.

Instead, tipping was traditionally reserved for service sector employees. For many of these workers, tips can be a significant portion of their income, especially since federal law requires employers of tip workers to pay just $2.13 in direct wages per hour.

Local customs and personal preference will affect tipping decisions, but here’s an overview of who and how much you should tip.

Who to Tip
When it comes to tipping, the guidelines are simple, according to 23-year-old etiquette expert Lisa Mirza Grotts. “A tip is required every time a service is rendered,” he says.

READ: How companies are pushing you to spend more.
This is how much tips restaurant waiters tip: According to Zietz, 20% of the bill at table restaurants and 10% at fast food restaurants has become the norm.