Long Running Transaction

 What do you do when you see long running transactions?

select sid, serial#, username, status, osuser, machine, terminal, last_call_et/60/60, to char(sysdate -(last_call_et/60/60/24), 'mm/dd/yy hh24:mi:ss'), to char(logon time, 'mm/dd/yy hh24:mi:ss'), module, program from v$session where status ='ACTIVE' and type!= 'BACKGROUND' and  last_call_et >= 30 order by sid;

I hope my query was right. last_call_et>= 30 minutes. Not sure if I need to express it as 30*60.

My question is whenevery there is long running transaction, what needs to be done? Look at undo space and see if the space needs to be increased or anything else? Thanks.