Death

Death occurs when Isaac runs out of health. There are multiple ways to cause this. As long as Isaac has health of any type (Red, Black, Soul, Bone or Rotten) left, they will not be killed, except if Isaac only has an Eternal Heart, though this is impossible.
 * Damage from monsters
 * Damage from bosses
 * Damage from obstacles
 * Damage from bombs
 * Damage from a Blood Donation Machine
 * Damage from Curse Rooms
 * Trading your last bit of health when making a deal with the Devil unless that item provides any hearts in return, such as the
 * Using against Satan
 * Using the card
 * ... and more.

Extra Lives
Typically, when Isaac dies, the game over screen appears; however, there are certain revival items, characters, and abilities that can resurrect Isaac after death. It is possible to obtain extra lives throughout a run by picking up various items. When Isaac dies, they will be revived in the previous room under the new life. Below is a list of these items and special death / extra life conditions that they bestow; take note that reviving as a different character than your current one gives any further Completion Marks to the new one, and the transformation may change stats if the new character has different base values. The order of revives is independent of order of item pickup, determined by the list below from highest to lowest placing:

Soul of Lazarus
When a is used, Isaac immediately dies and is revived in the same room at half a heart and given some invincibility time. Alternatively, if Isaac dies while holding the stone, it is automatically used upon taking the fatal hit, acting as another life (still has the same effect as if it was used manually). If used manually, it does not count as taking damage, so any positive or negative effects associated with taking damage are not triggered.

1up!
The grants Isaac an extra life. Upon death of the previous life, Isaac will be revived with full Red Heart containers. If he lacks any, he will revive with one Half Soul Heart. For characters that can't use Red hearts, they will respawn with 3 of their respective heart type (Soul Hearts for, , , , and , Black Hearts for and , and Bone Hearts for ).

Lazarus and Lazarus' Rags
While holding, Isaac will be revived as with 1 Red Heart container. Note that unlike other items, the Rags will revive Isaac on the spot with brief invincibility, as opposed to in the previous room.

Dead Cat
grants Isaac nine extra lives. Isaac will be revived with one Red Heart container unless playing as or, where he will be revived with one soul heart/black heart.

Inner Child
grants Isaac another life. Upon death, Isaac respawns in the same room with half a heart left, a massive size down, and +0.2 speed. If Isaac has any heart containers, he will respawn with half a heart container filled; otherwise he will respawn with half a soul heart.

Guppy's Collar
grants Isaac a 50% chance to be revived with one Red Heart container half-filled. If he lacks any, he will be revived at half a Soul Heart instead. If the collar fails to activate, any other items after Guppy's Collar will be checked. The revives from Guppy's Collar are seeded per death.

Ankh
The revives Isaac as ??? with 3 Soul Hearts.

Broken Ankh
When held, Broken Ankh has a 2/9 chance to resurrect Isaac as ??? with 3 Soul Hearts. The revives from Broken Ankh are seeded per death.

Judas' Shadow
revives Isaac as with 2 Black Hearts.

Missing Poster
While holding this trinket Isaac will be revived as and Missing Poster will be removed. If already playing as The Lost, Isaac will respawn as normal in the previous room.