A Practice Note explaining how a defendant in a Pennsylvania court of common pleas civil case may assert a claim against a nonparty (an additional defendant) that is or may be liable for losses related to the plaintiff's claims. Specifically, this Note discusses the types of third-party claims a defendant may plead as a joining party and identifies key considerations when drafting, filing, and serving a joinder complaint and related documents, including a praecipe for writ to join additional defendant. This Note also covers how parties may respond to third-party claims and reply after the additional defendant answers the joinder complaint.