If you are planning hiking trip ini Tenerife, Canary Islands, then make sure to plan well in advance as the permits go away veery quickly
Hiking treks that requires advance permits
Hiking treks in Tenerife that requires permits (they are all free as for now!), but do book well in advance where possible:
- Teide peak ascent
- Barranco de Masca trail
- Barranco del Infierno trail
- Sendero el Pijaral trail (aka Enchanted Forest) (Anaga)
- Anaga Monte de Aguirre Exclusion Zone trail (Anaga)
Read more in this link: https://smarttravel.family/tenerife/tenerife-hiking-routes-requiring-advance-permit/
Teide peak hike permission can be obtained from the link below (book more than 6 months in advance!):

My advice, book several days apart, so that if cable car is not working one day (or bad weather conditions), then you can go on another day. The only trick is that you also need to book cable car and they sell out too! So this will work only if you hike up and down all the way, which is a very strenous activity and you need to train for it and also come very well prepared (enough food, water and good hiking shoes, walking poles and very warm clothes).
We were available to book only via 3rd party getyourguide.co.uk, but the cable car was not working, so we lost our hike and got a full refund. The cable cars were offline for almost entire week due to the strong winds and snow! Talking about unlucky!
Masca Gorge Hike

Free permit for Masca Gorge (plan in advance) can be obtained in the link below (the new permits are released on Thursdays and Friday):
Another option is to book with an agency. I found that most of the agencies offering Masca hike, are booking through El Cardon agency, so probably best is to book directly from them. Make sure to plan it in advance!
If you find other direct agencies, please leave in the comments!

Also this agency, but it seems you must book more than 6 months in advance:

More useful information about Masca Gorge below.