Hair glue
Also called hair resin, it's generally the most used method for attaching a hair system, because it's super strong (depending on the type of resin) and also stays really comfy. Right now the most sticky and sweat-resistant hair glue is Power full, and you can order it via this link.
Order Power full glue
Hair adhesives
The cool thing is it's really easy to maintain and a whole lot simpler to apply than hair glue. We usually recommend this for lace bases like the Total HD for example. If you use it on polyurethane bases (Ultra HD), it won't be ideal if you want an undetectable result since it'll add a tiny thickness to the base of the system.
Check out this blog post where we explain all the benefits of Power full glue in detail.
You should do a skin test for 48h by applying a drop of hair resin behind your ear.