A deadlock is a special form of blocking that occurs when two or more transactions each have locks that the other needs. Since neither transaction can release their locks until the transaction is complete, SQL Server is forced to roll back one of the transactions. An error is returned naming the rolled-back transaction as a deadlock victim. This is fundamentally a performance problem, but one that is exacerbated by the need to roll back the transaction and then resubmit the query for processing a second time. Dealing with deadlocks can be challenging.