Sometimes fate blesses or curses a combatant, causing the novice to hit and the veteran to miss. If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the target's AC. In addition, the attack is a critical hit, as explained later in this chapter.

- "Rolling a 1 or a 20", Player's Handbook p194

When you score a critical hit, you get to roll extra dice for the attack's damage against the target. Roll all of the attack's damage dice twice and add them together. Then add any relevant modifiers as normal. To speed up play, you can roll all the damage dice at once.

- "Critical Hits", Player's Handbook p196

Spell attacks and weapon attacks both have a chance of scoring a critical hit.

/- Jeremy Crawford, February 8 2018

Spells attacks most certainly can crit. A save based spell like Fireball cannot, but spell attacks like Firebolt or Eldritch Blast can.