To boot your Intel-based Mac into any of OS X’s built-in startup modes, you must press and hold one or more keys detailed below after turning on the computer. Press the keys immediately after hearing the startup sound, unless otherwise specified.
Manually restart: Power (⎋)
Boot from CD/DVD: C
Startup Manager: Option (⌥)
Single-User Mode: Command (⌘)—S
Target Disk Mode: T
Verbose Mode: Command (⌘)—V
Safe Mode: Shift (⇧)
Recovery OS: Command (⌘)—R
Internet Recovery OS: Command (⌘)—Option (⌥)—R
Apple Hardware Test: D
Apple Hardware Test over the Internet: Option (⌥)—D
Boot from NetBoot server: N
Boot from NetBoot server using the default image: Option (⌥)—N
Eject removable media: Media Eject (⏏), F12, mouse button or trackpad button OS X Startup Volume: X will start up your Mac from an OS X startup volume when it’d otherwise boot from a non-OS X volume, like a Windows partition
Prevent Login Items from opening: To override your current settings and prevent Login Items and Finder windows from your last session from opening when you log in, hold down the Shift (⇧) key after clicking the Log In button in OS X’s login window until the desktop appears.
Prevent automatic login: If you enabled the option to automatically log a specific OS X account every time your Mac starts up, you can override this setting by holding the left Shift (⇧) key after OS X’s boot progress indicator appears.
Reset PRAM/NVRAM: Command (⌘)—Option (⌥)—P—R, then release the keys after you hear the startup sound for a second time
Reset SMC: Shut down your Mac, and then press and hold the left-side Shift (⇧)—Control (⌃)—Option keys along with the Power (⎋) button, all at the same time, until the computer turns on. Now release the keys and then turn your Mac back on using the power button.