A character can try to do anything you can imagine, just as long as it fits the scene the GM describes. Characters accomplish things by making skill checks, attribute checks, or attack rolls. While the rules for making these checks follows the core mechanic, each type of roll has a slightly different purpose.
To make a skill check, roll a d20 and add your character's skill modifier for that particular skill. Compare the result to the Difficulty Class (DC) for the task at hand. The DC may be determined by the skill's description, or set by the GM, or established by another character's check result. More information can be found on the Skills & Talents page.
Unopposed Checks: An unopposed skill check's success depends only on your character's actions.
For Example: If Behzed tries to climb a chain to get out of underground vault, the result depends only on his skill and luck, so an Athletics check is made. If his result is equal to or higher than the DC, then the attempt succeeds.
Opposed Checks: An opposed check is used when another character actively attempts to prevent your character from succeeding at a given task.
The DC for an opposed check is the check result of the character opposing your action. The opponent's check might be made using the same skill as you are using or a different skill, as defined in the skill description. The character with the higher result succeeds at the action being attempted, while the character with the lower result fails. In the case of a tie, roll the checks again until one character achieves a higher result than the other.
For Example: Behzed is trying to hide from an opponent, so his Sneak check is opposed by the opponent's Awareness check. If Behzed's result is higher, he successfully hides, and his opponent fails to spot him. If the result is lower, then Behzed's opponent spots him.
Attribute checks are used when a character doesn't have any ranks in a particular skill and tries to use that skill untrained. An attribute check is really just a special case of a skill check with a +0 skill modifier. Some skills can't be used untrained, however; see the Skill Summary for details.
For Example: If Behzed wants to fast-talk a couple of town guards but doesn't have any ranks in the Bluff skill, the GM can call for a Charisma check. (Charisma is the attribute associated with the Bluff skill, and the Bluff skill can be used untrained.)
To make an Attribute check, roll a d20 and add your character's modifier for the appropriate attribute. If the result is equal to or greater than the task's DC, the check succeeds.
At the heart of the d20 combat system stands the all-important attack roll. To attack an opponent, roll a d20 and add your character's attack bonus. If the result equals or exceeds the opponent's Defense, your attack succeeds.
On a successful attack, you roll to determine how much damage your attack deals to the opponent. Roll the type of dice indicated for the weapon used and add any modifiers that apply.
For Example: If Behzed successfully attacks an opponent with his pistol, he deals 2d6 damage to the foe. If Behzed is using his crowbar, a successful attack deals 1d4 damage plus his Strength bonus.
Damage reduces hit points. When all of a character's hit points are gone, the character falls unconscious and is dying.
A critical hit (a potent result on an attack) deals double the amount of damage (plus it has the potential of inflicting a disabling injury if the optional Critical Effects rules are used). An attack inflicts a critical hit if the attack roll is a natural 20, as long as the result would normally succeed. If the attack would normally miss even on a 20, it is treated as a regular hit instead of a critical. (Note that this is a change from the standard d20 rules. In Ground Zero, critical hits do not have to be confirmed.) Some attacks inflict critical hits on natural rolls of 19-20 or other values.
Important: There are two types of attacks in the game: melee and ranged. Melee is a fancy French word for hand-to-hand combat, such as with a club, a knife, or a character's fists. A ranged attack uses a weapon that attacks opponents at a distance, such as a rifle or a grenade. Your character will probably have a different attack bonus for each type of attack.
Your hit points (hp) tell you how much punishment you can take. Each character can withstand a certain amount of damage before becoming fatigued, getting knocked unconscious, or dying. The amount of hit points you have are based on your Constitution score, genotype, and class, and that number increases with each level gained. (Characters in Ground Zero start with more hit points than in other d20 games, but gain fewer hp over time.)
The saving throw is a die roll that gives a chance, however slim, that the character or creature finds some way to save himself from harm or at least lessen the damage. To make a saving throw, the player rolls his luckiest d20, adds all relevant modifiers, and hopes the result is equal to or greater than the target Difficulty Class of the save. Saving throws come in three flavors: