Spell penetration

Spell Penetration check
Spell Resistance (SR) is a special anti-spell defense possessed by some creatures. Spell Resistance is an additional layer of defense against most non-damaging magical attacks, in that it adds an extra roll that the caster has to make for his magic to "land" on that target.

Spell Resistance only matters if the spell's creature actually has it, and the spell being cast is one that can be stopped by Spell Resistance. In DDO, spells that most purely cause damage ignore Spell Resistance.

Spell Penetration is the act of trying to penetrate that defense. Spell penetration roll is a caster level check, i.e. you roll 1d20 + caster level. This roll is modified by additional bonuses listed below.

Spell Penetration feats

 * Spell Penetration feat gives +2 to die roll
 * Greater Spell Penetration feat gives another +2 to die roll
 * Epic Spell Penetration feat gives another +4 to die roll
 * Past Life: Wizard grants +2 to the die roll, it can be acquired up to 3 times for a maximum of +6
 * Past Life: Favored Soul grants +1 to the die roll, it can be acquired up to 3 times for a maximum of +3

Spell Penetration enhancements

 * Spell Penetration class enhancements for Archmage, Angel of Vengeance, Spellsinger, or Tainted Scholar give +1, +2 or +3 to die roll.
 * Wizards can also gain +1 or +2 to the die roll from Pale Master core abilities, and +4 from Knight's Arcanum in the Eldritch Knight tree.
 * Spellsingers actually have two enhancements, one that boosts either their own or others' up to +3, and one that gives themselves up to +2.
 * Epic Destiny Abilities in Magister, Draconic Incarnation and Exalted Angel can also grant up to +3.
 * Elves, Dragonborn and Sun Elves can get up to +3 in racial enhancements with the Arcanum enhancement

Item enchantments
The Spell Penetration item enchantment gives an equipment bonus to spell penetration checks. Notable sources:
 * Named items: +1 to +9
 * Randomly generated caster weapons: +1 to +7
 * Cannith Crafted Items: +1 to +6
 * Crafted Thunder-Forged items: +7 (Tier 2 upgrade)
 * Slave Lords Crafting: +7 (Legendary version)
 * Topaz of Spell Penetration: +1 to +5
 * Historical: Previously, an item's Spell Penetration bonus applied only for spells up to a given level, for instance a Spell Penetration III Dagger would give a +2 bonus for spells of the levels 1, 2, and 3. Items with these enchantments have been retroactively converted to new format.

Insight bonus

 * Random generated items: +1 to +4
 * Cannith Crafted items: +1 to +3
 * Named items +1 to +5

Other bonuses

 * Quality bonus on named items: +1 to +3
 * Eye of the Beholder three piece filigree set bonus (sentient weapons)

Effects which increase your caster level also increase your spell penetration checks.
 * Arcane Augmentation
 * Illusory Death

Maximum spell penetration
Breakdown as of Update 35:

Favored Soul
 * 20 Levels

Enhancements
 * 3 Sun-Elf, Elf, or Dragonborn (Arcanum)
 * 4 Angel of Vengance (Spell Penetration) Note: Bards get 5 from Spellsinger Tree. Warlock and Wizard get +3 from Tainted Scholar and Archmage, respectively.

Items
 * 7 Thunder-Forged Tier 2 or Slave Lords Crafting
 * 4 Insightful Bonus (Item:Legendary Mutilator of Minds)
 * 2 Spell Augmentation (Item:Gauntlets of the Arcane Soldier)

Epic Destiny Abilities and Twists
 * 5 Epic Destiny Caster Levels
 * 3 Piercing Spellcraft (Magister)
 * 3 Piercing Spellcraft (Exalted Angel)
 * 2 Echoes of Ancestor: Wizard

Feats
 * 2 Spell Penetration
 * 2 Greater Spell Penetration
 * 4 Epic Spell Penetration
 * 61 Spell Penetration before Past Life Feats (62 if Spellsinger)

Past Life Feats 
 * 6 Wizard Past Lives
 * 3 Favored Soul Past Lives
 * 70 Spell Penetration with Past Life Feats (71 if Spellsinger)

Temporary Buffs
 * 3 Spellsinger Tier 4 Spell Penetration - Inspire Competence
 * 73 Spell Penetration with Past Life Feats and temp buffs (71 if Spellsinger - Already counted above)