Shields
Shields can range from a dueler’s fencing buckler to the tower shields of mercenary armies. You may wield two shields, but only gain the defensive benefits of one shield at a time. When it is used to attack, a shield is treated as an improvised weapon that deals 1d4 bludgeoning damage.

Table: Shields

Donning a shield grants no benefit to Armor Class if you are not proficient with shields, and you are unable to take cover behind it, plant it on the ground, or sacrifice it.

A creature one size category larger than the shield was designed for treats it as a shield that is one degree lighter, while a creature one size category smaller treats it as a shield that is one degree heavier. If that would make a light shield lighter or a heavy shield heavier, the shield cannot be used by that creature.

The costs and weights in Table: Shields are for shields made from steel. 

Light. Light shields increase your Armor Class by 1. You may throw this shield, treating it as an improvised weapon that deals 1d6 bludgeoning damage.

Medium. Medium shields increase your Armor Class by 2.

Heavy. Heavy shields increase your Armor Class by 2 and you gain an expertise die on Dexterity saving throws . When you take the Dodge action while wielding a heavy shield, you may instead take cover behind your shield, gaining a knack die to your Armor Class until the start of your turn. You have disadvantage on Dexterity (Acrobatics) and Dexterity (Stealth) checks while wielding a heavy shield, and you cannot squeeze through spaces smaller than your size category.

Pavise. These large rectangular shields share the properties of a heavy shield. These shields cover the entire body and reduce your Speed by 10 feet. On your turn, you may use an object interaction to plant it in the ground, gaining half cover (+2 bonus to AC, Dexterity saving throws, and ability checks made to hide) and advantage on saving throws made to resist being shoved or knocked prone and while you remain behind it. Unplanting a pavise requires a bonus action.

Pavises are bulky items and count as such even when donned.

Improvised Shields
If you are proficient with shields, you can use an action to pick up a nearby object of your size category or larger to use as a shield. The size of the object determines the degree of shield it becomes: an object your size can be used as a light shield or an object of one size category larger can be used as a medium shield.

An improvised shield is destroyed when you take a critical hit (it cannot be sacrificed), it cannot be repaired or customized, and it becomes useless after 10 rounds of combat.

Sacrifice Shield
When you take a critical hit, you can use your reaction to block it and sacrifice your shield, turning the critical hit into a regular hit. Afterward, your shield is broken, or if your shield is magical it instead becomes mundane for 1 hour.