How does one monitor Parallel Query Execution?

Use the following Oracle data dictionary views to monitor parallel queries:

Prior to Oracle 8i:
SELECT * FROM sys.v_$pq_sysstat;
Oracle 8i and above:

select * from v_$pq_sysstat;
SELECT * FROM v_$px_process;
SELECT * FROM v_$px_sesstat;
SELECT * FROM v_$px_process_sysstat;