Apple’s new mobile payment system, Apple Pay, officially rolls out today.

Apple Pay was announced last month when Apple, No. 2 in the 2015 Internet Retailer Mobile 500, unveiled the new iPhone 6, iPhone 6 Plus and Apple Watch—all of which are outfitted with a fingerprint scanner and near field communication, or NFC, chips that allows the payment system to work. A consumer pays by placing her phone near a payment terminal that includes the wireless NFC technology, then authenticates her identity by placing a finger on the phone.

Last week, Apple also announced the new generation of iPads would have the fingerprint scanner but not NFC chip. That means consumers can use their iPads to make purchases in mobile apps that have incorporated the Apple Pay system, but not in stores.

A representative at a Chicago-area Apple Store says Apple Pay won’t be available to consumers until Apple’s new iOS 8 mobile operating system gets pushed out later today. Apple Pay will be included as a free update to that operating system. Consumers will then have to set up the service.

At least 36 retail and restaurant chains will accept Apple Pay in physical locations, and another 23 in mobile apps. To use Apple Pay, a consumer takes a picture of her credit card in the Apple Passbook wallet, which then asks her to authenticate herself with the bank that issued the card. Then Passbook stores information about the card on an encrypted secure element within the smartphone. Passbook does not store the actual card number or any identifiable information on the phone. Instead, Apple stores a device-only account number. Passbook generates for each payment a one-time number with a dynamic security code. Apple says even if an iPhone 6 is lost or stolen, there is no need to cancel a credit card. The company also says if criminals were to break into a retailer’s payment system they would not be able to use the Apply Pay information they obtain to commit fraud.

Among the initial retailers rolling out Appley Pay is Walgreen Co., which plans to accept the mobile payment system in   more than 8,200 Walgreens and Duane Reade drugstores. Walgreens is No. 43 in the Internet Retailer Top 500 Guide. High-end grocery store chain Whole Foods is rolling out Apple Pay to all of its 381 stores in the U.S, the company announced today. Office Depot Inc., No. No. 9 in the Top 500 Guide, also plans to roll out Apple Pay to all of its stores in the U.S. The company also says it plans in 2015 to add Apple Pay to its apps for the iPhone and iPad.

Not all retailers are planning a complete Apple Pay rollout right away. RadioShack Corp., for example, is initially rolling out Apple Pay in 900 stores in five markets—Manhattan; Washington, D.C.; Chicago; Los Angeles; and San Francisco—though the company plans to add markets in 2015. RadioShack is No. 317 in the Top 500 Guide.

Target Corp., No. 37 in the Mobile 500, plans to accept Apple Pay in its mobile apps but not in stores. A company spokesman said earlier that Target app users who have an iPhone 6 or 6 Plus and an Apple Pay wallet set up will see an express checkout button on product detail and shopping cart pages. With a click of a button and a fingerprint verification, the purchase will be complete. The company offered no further comment this morning.

Staples Inc., No. 12 in the Mobile 500, also plans to accept Apple Pay in its mobile apps, and a company spokesman said more information would be available later today.

The list of retailers accepting the payment method in stores and in apps has grown since the initial announcement. Here is a full list of brands accepting Apple Pay in stores:

Aeropostale

Apple Store

American Eagle Outfitters

Anthropologie

Babies ‘R’ Us

BJ’s

Bloomingdale’s

Champs Sports

Chevron

Disney Store

Walt Disney World Resort

Duane Reade

ExtraMile

Foot Locker

Footaction

Free People

Macy’s

McDonald’s

Nike

Office Depot

Panera Bread

Petco

Petsmart

RadioShack

Sephora

Six:02

Sports Authority

Staples

Subway

Texaco

Toys ‘R’ Us

Unleashed

Urban Outfitters

Walgreens

Wegmans

Whole Foods Market

Full list of brand’s accepting Apple Pay in apps:

AirBnB

Chairish

Disney Store

Fancy

Groupon

Hotel Tonight

Houzz

Instacart

Jackthreads

Levi’s Stadium

LIFX

Lyft

MLB.com

OpenTable

Panera Bread

Sephora

Spring

Staples

Starbucks

StubHub

Target

Tickets.com

Uber

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