The steps to install Halo: The Master Chief Collection (MCC) are different depending on which platform you're playing on. Make sure you check the correct section below.

Xbox Game Pass (on PC)

If you are an Xbox Game Pass for PC or Xbox Game Pass Ultimate subscriber, you can play Halo: The Master Chief Collection including Halo: Reach and Halo 3: ODST on your PC through the Xbox PC app. Here's how.

1. Download the Xbox app for PC and launch it.
2. Sign in using your account with an active Xbox Game Pass subscription.
3. Type "Halo" in the search bar in the top right of the app and search.
4. Select Halo: The Master Chief Collection from the search results.
5. Click the Install button.

Xbox Game Pass (on Console)

If you are an Xbox Game Pass subscriber, you can play Halo: The Master Chief Collection as part of your subscription. This includes the Halo: Reach DLC. To play:

1. Sign in to your Xbox One or Xbox Series X|S console using your account with an active Xbox Game Pass subscription.
2. Press the Guide button (the Xbox logo button in the center of the controller) and navigate to the "Store."
3. In the store, press Left Bumper (LB) to navigate to the "Game Pass" menu bar
4. Navigate to Halo: The Master Chief Collection and press A.
5. Select Install from the bar that pops up.

Xbox One and Xbox Series X|S Console

This section applies to people who own Halo: The Master Chief Collection without Xbox Game pass.

If you already own Halo: The Master Chief Collection on Xbox One, Halo: Reach and Halo 3: ODST will be listed as separate DLC items for purchase in the Microsoft Store.

On your Xbox One console:

1. Navigate to the Halo: The Master Chief Collection tile in your Games & Apps and press the menu button. 
2. Scroll down to Manage game & add-ons and press A
3. On the left-hand side, scroll down to Ready to install and press A
4. Select your choices on the right and install them.

Steam

When you purchase the game, the store will automatically prompt you to install it.

If you need to install the game any other time:

1. Sign in to Steam using the account that purchased the title
2. Select Library from the top navigation bar in Steam.
3. Locate the game on the left-side navigation bar, right-click it, and select Install.

Microsoft Store (PC)

When you purchase the game, the store will automatically prompt you to install it.

If you need to install the game any other time: 

1. Check to make sure you are signed in to your PC using the correct account.
2. On your keyboard, press the Windows key and type "Store" in the search bar to bring up the Microsoft Store app.
   • You can navigate to or open the Microsoft Store however you like; this is just one way to do it.
3. In the Windows Store app, navigate to the top right and click the [...] next to your account's profile picture
4. Select "Downloads and updates"
5. On the left-hand side, select "Ready to install"
6. Find the game in the list and select "Install"