Objective:

Take a picture of what is displaying on my phone
Take a screenshot on my Android smartphone

Environment:

Android 4.0 and up

Additional Information:

Instructions for iOS Users can be found here.

Every Android phone is different, and so is taking screenshots with them.

Android 4 introduced simple screenshots for all with the handy power-and-volume-down-key combo, however, nailing it can still be tricky. That's why some phone makers have introduced new methods.

If the power-and-volume-down-key combo doesn't work for you, simply Google the term "screenshots [phone name]" and see if the manufacturer included a secondary way to take screenshots.

Procedure:

General Method (Android 4.0 and up—most phones made within the past 5 years):

Hold down the power and volume-down buttons for a couple of seconds.
The screen will flash and you may hear the camera shutter sound.

Google Assistant (Android 6.0 and up; must be enabled)

Say "OK Google, take a screenshot."
- If you're having trouble, make sure to listen for the "ding" sound effect after you say "OK Google", to ensure it's listening before you say the rest of your command.
It will save a picture of the screen below the Assistant interface. If you want to capture the Google Assistant screen itself, you'll have to use the hardware buttons.

Samsung Galaxy S7 and earlier:

Hold down the home and power buttons for a couple of seconds.

Sony Phones:

Hold down the power key until a screen appears and tap Take screenshot.

HTC 10:

Hold down the home and power buttons for a couple of seconds.

Acer & Asus Phones:

Pull down the notification panel to reach quick settings and tap the Screenshot icon.