Pleadings may be amended once as of right within a short window and thereafter with the opposing party’s consent or leave of court, which is freely given when justice requires.
Grounds to deny. Undue delay, bad faith, repeated failure to cure deficiencies, undue prejudice to the opposing party, and futility. Prejudice is the dominant consideration, and it grows as the case matures.
Scheduling order deadlines. After the deadline to amend has passed, the movant must generally show good cause for modifying the schedule — focused on the movant’s diligence — before the liberal amendment standard applies.
Relation back for claims. An amendment relates back to the original filing where the claim arose out of the conduct, transaction or occurrence set out in the original pleading. This saves claims that would otherwise be time-barred.
Relation back for parties is stricter. The claim must arise from the same transaction, and within the period for service the new party must have received notice such that it will not be prejudiced, and must have known or should have known that the action would have been brought against it but for a mistake concerning identity. A deliberate choice to sue one defendant is not a mistake, which is why substituting a properly identified party after limitations usually fails.
Doe defendants are treated differently by state and federal practice.