Tax relief on pension contributions is one of those rare occasions when the taxman gives you something back.
In 2012-2013 it cost the government nearly £35 billion.
This is because, under current rules, when you pay money into a pension, the government effectively pays 20% of the total contribution (subject to maximum limits).
If you are a higher tax payer then the government could in effect contribute 40% or even 45% in total.
But it should always be remembered that the value of any relief will depend on your individual circumstances and also, tax rules can change.