ORA-12154: TNS:could not resolve the connect identifier specified

Scenario: You just installed Oracle Developer Tools, and now you cannot connect to the database using a Oracle connect alias that has worked in the past. You get a ORA-12154 error when you attempt to connect.

Cause: When you install Oracle Developer Tools, it typically creates a new "Oracle Home" (a new Oracle subdirectory, for example C:\oracle\product\10.2.0\my_new_home ) Each Oracle home maintains it's own copy of your connect aliases in a file called TNSNAMES.ORA. When you try to connect from this new Oracle home, none of your old connect aliases will exist in the new copy of TNSNAMES.ORA, therefore you get this error. They need to be copied over to the new Oracle home.

Solution: Copy <OLD_ORACLE_HOME>\NETWORK\ADMIN\TNSNAMES.ORA to <NEW_ORACLE_HOME>\NETWORK\ADMIN\TNSNAMES.ORA .
In some configurations, copying SQLNET.ORA and LDAP.ORA files from the same directory above may also be required.

Additionally:

64-bit Windows users who after doing the above steps still get ORA-12154 should either: 1) Upgrade to the latest version of their Oracle client that contains the fix for bug 3807408, or 2) rename subdirectories in their application that contain parenthesis to remove those parenthesis, in particular: C:\Program Files (x86)\Microsoft SQL Server and C:\Program Files(x86)\Microsoft Visual Studio 8