How to get IMEI number of android device?

IMEI number is another way of uniquely identifying cellular devices.
 
Each cellular device has a unique IMEI number which can never clash with another device’s IMEI number.
 
Android devices [phones only] also have International Mobile Station Equipment Identity or IMEI number.

And you should add the following permission into your Manifest.xml file:
<uses-permission android:name="android.permission.READ_PHONE_STATE"/>

TelephonyManager tm = (TelephonyManager).getSystemService(Context.TELEPHONY_SERVICE);
String imei = tm.getDeviceId();