The EU ETS is divided into different trading periods or allocation periods. Each period is characterised by specific legal regulations concerning the free allocation of EUA, which have developed over the years.

• 1st trading period 2005-2007
• 2nd trading period 2008-2012
• 3rd trading period 2013-2020
• 4th trading period 2021-2030

While in the first three trading periods the trading period corresponded to the allocation period, the fourth trading period is divided into two allocation periods (1st period: 2021 - 2025, 2nd period: 2026 - 2030). At the beginning of each allocation period, the free allocation quantity to a plant operator is determined.

In addition to the possibility of saving a surplus of EUAs within a trading period, the transfer of EUAs to the following allocation periods has also been permitted since the second trading period. This possibility of transfer is called Allowance banking.
Within an allocation period, the EUAs allocated for the current year can also be used for the   previous year’s levy.

An EUA generally never loses its validity or value unless it is issued to compensate for CO2 emissions. EUAs, which are deleted under the market stability reserve, are an exception.