• The one point in improved Exhaust gives you more debuff power, which is Trundle's strength, and it works well in his kit. If you don't run Exhaust, don't take it, since improved Ignite isn't great on him and it obviously doesn't benefit Heal at all. 

• The points in Durability give him health per level, which is one of his most desired stats and helps him scale better into the late game. Taking it also allows you to put a point in Veteran's Scars which is a very valuable mastery because of how many resistances you should have between runes and masteries.
• Hardiness and Resistance both are pretty self explanatory, especially considering how much you invest in resistances in runes already.
• Unyielding and Block both are incredibly strong masteries, especially in the laning phase on a support such as Trundle that wants to avoid as much damage as possible. They stack up pretty quickly with all your resistances and health.
• Relentless and Tenacious both make it more difficult to kite you, which is a huge weakness Trundle has as a support. They make it possible for him to bypass those weaknesses and still reach his target to use Chomp (Q).
• Safeguard is actually a very important one point mastery as it makes tower dives much safer, as generally you should be tanking them even if your jungler is diving as well.
• Juggernaut improves his health late game pretty significantly, making it a must on Trundle whether you're support or in any other role.
• Defender and Legendary Armor both work incredibly well with your runes and the previous masteries, making him even tankier.
• Reinforced Armor is similar to Unyielding and Block, but is based off a percent amount, making it a very strong late game point. It's also useful against carries like Draven early game.
• Lastly, Honor Guard on top of all that makes him take very little damage, allowing him to shrug any off and heal it back up with his passive and ultimate.

• The only reason you would take this is to get some bonus damage and cooldown reduction (getting Sorcery, Deadliness, and Weapon Expertise) while still remaining pretty tanky. It's one of the better alternatives, and is great against other melee supports and/or short ranged carries.

• This gets you relatively tanky while still picking up some nice utility. However, Trundle doesn't really need the mana, the cooldown reduction is too far into the tree, and the movement speed isn't that strong. The only benefit from going this route is it allows you to start with bonus gold and gold per ten, and gives you more sustain and sight through Biscuit and Explorer Ward.

• I actually prefer this over the previous one even though you're less tanky. The few points you don't have in the defense tree doesn't make a huge difference in the long run, but being able to get cooldown reduction, movement speed, and the other benefits mentioned in the previous paragraph makes it worth it. Also, even though you won't be able to use Pickpocket that frequently, it still helps and it only costs one point. If you're going to go that deep into the utility tree, you might as well take it.

• This is an extremely high risk, medium reward setup. You can get poked down easily if you take this setup, and your all-in is only slightly better. I'd rather go into the defense tree or utility tree and adjust my runes for more damage (taking armor penetration marks or something).

• Honestly, one of the worst alternative setups. You're getting tons of cooldown (both in the offense and defense tree), as well as starting gold, gold per ten, Bicuit, Explorer Ward, and some damage, but if you ever go in to use Chomp (Q) on the enemy carry or support, you're most likely going to die.

First Few Times Back

  - If you're really struggling in lane, you can buy a Doran's Shield and/or Ninja Tabi, which will both greatly reduce the amount of damage you take from the enemy ADC. The both work incredibly well with Trundle's runes and masteries, making him nearly unkillable. Only buy those if you're taking heavy damage, though. Otherwise...

 If you have the gold, try to get an early Sightstone and then a Philosopher's Stone, and afterwards build a Kindlegem as soon as possible. Sightstone is the most efficient gold per ten item currently (while it doesn't GIVE gold per ten, it provides free wards, making it more effective than other support items by far), and Trundle really benefits from the health it gives. All of the stats on the items work extremely well on Trundle, but the most important ones that you should focus on are health and cooldown reduction.

  - Basically every item you get on Trundle builds out of Ruby Chrystal, so buy those with vision (pink) wards whenever you can afford them.

   - Early Mercury's Treads are great since you should be tanking and the tenacity really helps him out. Boots of Mobility and Ninja Tabi are also good on him.

 - Ionian Boots of Lucidity should only be picked up if you're having trouble buying cooldown reduction early on, but if you get them, sell them later in favor of one of the others.

Late Game

  - If the opposing team has a large amount of physical damage, you should buy Iceborn Gauntlet and Randuin's Omen. The first gives armor, cooldown reduction, damage, and a slow, while the second gives health, armor, and the passive and active slows. All of them benefit Trundle greatly and improve his peeling power.

  - However, if the enemy team has more magic damage than physical, Frozen Mallet and Runic Bulwark are the best options. Mallet gives health, a little bit of damage, and constant slows on each auto attack, and Bulwark gives you health and your entire team magic resist and armor.

 - If they have mixed damage, you can opt for Iceborn Gauntlet and Runic Bulwark. This provides you with the utility of Gauntlet and the defensive aura from Bulwark. While this means you won't have a strong health item, these will benefit your team the most in this situation. You will still have health from four out of six of your items, and stacked onto your resistances and your ultimate, that's still nothing to laugh at.

Situational