Beast Mastery

Beast Mastery is a Ranger secondary attribute, meaning that any character with Ranger as their primary or secondary profession can put points into this attribute.

Guild Wars Description
"Beast Mastery increases the damage dealt by your animal companion and it's chance to inflict a critical hit. Many Ranger skills, especially those related to making your animal companion stronger, become more effective with higher Beast Mastery."

General Description
Beast Mastery is the main attribute for enhancing the power of your pet. Many of the skills focus on directing your pet to attack your opponent, disrupting their actions, providing your pet with defense and healing, or symbiotically linking your well being to your pet's. Follow the link for an in-depth analysis.

Pets can also be a source for corpse explotation since it is not always critical to keep your pet alive and resurrecting the pet can be fairly cheap and quick.

Beast Mastery also consists of Nature Rituals that provide a global effect to all non-spirit creatures. Many of the rituals focus on health manipulation, increasing health or reducing healing for example. Strategically, this can be used to the player's advantage since both the player and the pet are able to reap the benefits. In a large pet group, these nature rituals can make a large difference as their effects influence more creatures in the player's party.