Before starting, make sure that USB DEBUGGING ENABLED in the settings of your phone !!!
1) BASIC STEP - connect the device via USB, then go to the page on the Android developers blog . There you will find the necessary information on adding USB vendor identifiers. Add your device identifiers and restart eclipse if necessary.
2) If you were able to see the connected device (using the command: 'adb devices' ) earlier, but no more than that, just try restarting ADB. (you can use the commands: 'adb kill-server' , and then 'adb start-server' . adb commands should be executed from the platform tools folder in the Android SDK if you did not export it).
3) If none of them work, and you are on a Windows machine, check that the installed USB drivers are correct. If you do not install the correct drivers For more information on installing and updating drivers, see http://developer.android.com/tools/extras/oem-usb.html
If this also does not work, try installing the Windows Universal ADB driver https://plus.google.com/103583939320326217147/posts/BQ5iYJEaaEH
4) You can also try to increase the timeout Go to settings → android-> DDMS in eclipse, and then try to increase the value "Timeout for connecting to ADB (ms)"
Update based on newer answers:
5) Run> Run Configurations> Target. Make sure that the option "Always request a device" is enabled.
Special case: Windows 8 and Nexus 10 (from this question: ADB No devices found )
Windows 8 does not recognize my Nexus 10 device. Fixed by installing (PTP) through the settings dialog on the device.
Settings> Storage> Menu> Connect a USB computer to "Camera (PTP)"
afadfadf Nov 09 2018-11-11T00: 00Z
source share