In many countries, it is necessary to pay patent renewal fees periodically in order to maintain a granted patent in force. In the UK, these renewal fees are payable annually, by the end of the month in which the respective anniversary of the filing date falls. However, some territories have less frequent renewal fees or allow renewals fees for some or all years to be paid at once.
With European patent applications (and patent applications in a number of countries around the world) it is also necessary to pay renewal fees periodically not just after grant but during the patent application procedure.
If renewal fees are not paid in time, there is typically a period of time (a grace period) during which they may be paid late, with a surcharge. If the renewal fee is not paid within the grace period, the patent expires.
In the UK, it is sometimes possible to restore a patent which has expired due to non-payment of a renewal fee, but only in limited circumstances, where the non-payment of the renewal fee took place despite all due care having been taken. It should be noted that third parties who have in the interim, in reliance on the renewal fee not having been paid, started serious and effective preparations to carry out actions within the scope of the patent in the UK may in some circumstances be entitled to continue those actions.