两者均指与诉讼紧密相关的当事人。区别在于indispensable party (必不可少的当事人）指的是由于其权利与诉讼主张紧密相关，故任何判决都势必影响到他（她）的利益，如果没有其作为共同当事人的参与，必将会导致诉讼被驳回（dismissal of the action) [1]。相比之下，necessary party (必要当事人）与诉讼的关联没有indispensable party紧密，尽管他（她）的参与会对完善解决诉争起作用，但如果其有合法的理由而不作为共同当事人参与，不会导致诉讼被驳回[2]。

[1]“A party who, having interests that would inevitably be affected by a court's judgment, must be included in the case. If such a party is not included, the case must be dismissed. ”Cf. Bryan A. Gamer, Black's Law Dictionary, 7th edition, at p. 1144, West Group (1999).

[2]“A person or entity whose rights are sufficiently involved in an action to require joinder as a party if that is possible；but if it is not possible, the action will be allowed to proceed without that additional party. ”Cf. James E. Clapp, Random House Webster's Dictionary of the Law, at p. 97，Random House (2000).