If you're running Mac, as @pedro mentions ensure you have the HAXM installer dowloaded via the Android SDK Manager.
Next install it! In finder navigate to /YOUR_SDK_PATH/extras/intel/Hardware_Accelerated_Execution_Manager/
Run and install the .mpgk in the following .dmg
IntelHAXM_1.1.0_for_10.10.dmg
IntelHAXM_1.1.0_below_10.10.dmg
Example:
$cd /YOUR_SDK_PATH/extras/intel/Hardware_Accelerated_Execution_Manager/ $open IntelHAXM_1.1.0_below_10.10.dmg