A motion to intervene asks the court to let a non-party join an existing lawsuit, either because they have a direct legal interest at stake or because their participation would help resolve related questions the case raises.
Intervention as of right vs. permissive intervention
Intervention as of right applies when the movant's interest could be impaired by the case's outcome and existing parties won't adequately represent it. Permissive intervention is left to the court's discretion, typically where claims share common questions of law or fact.
Show the interest is direct, not general
Courts look for a concrete legal interest tied specifically to the case — a general policy concern or indirect economic effect usually isn't enough to justify intervention as of right.
File promptly
Timeliness is a factor courts weigh heavily. A motion filed after significant progress in the case — discovery closed, motions decided — faces a much higher bar than one filed early.
Attach the proposed pleading
Most jurisdictions require the motion to attach the pleading the intervenor would file if allowed to join — this shows the court exactly what the intervention would add to the case.