Release clauses are not a legal requirement in La Liga. The Real Decreto 1006/1985 established release clauses but also made it clear that not having a release clause is allowed.

Article 16 lays out the process if a player without a clause wants to leave. The Labor Court would determine the value. Before that process can even begin a buying club would first have to agree to pay the fee before it’s even set.

The fee would be determined by several factors which essentially would make it fairly high. The entire process makes it extremely unlikely for a player to leave, but it’s not impossible.

If a club outside the country wants to go through the process it’s even more complicated. In addition to not knowing the fee beforehand they would have a sanction where the player couldn’t play for 4-6 months.

To put it simply, not having a release clause doesn’t make it impossible for a player to leave, but it does make it so difficult that other clubs essentially won’t bother with it. By signing a contract without a clause players are very much guaranteeing their future at the club.

As of now, six Athletic players have contracts without release clauses. Iker Muniain, Óscar De Marcos, Mikel Balenziaga, Ibai Gómez, Raúl García, and Unai Simón.