Abstract Two methods are today available in the manufacture of seamless tubes of 4 in. and greater diameter. These are the pilger process and the automatic or plug-mill process. The cost of installation is about the same for either process. The cost of tool equipment is much greater for the pilger process and it is claimed that the quality of the tube, particularly in the matter of service, is more uniformly reliable with the plug rolling process than with the pilger process. Referring particularly to the plug rolling process the authors discuss the difficulties of operation and modern improvements aimed at obviating these difficulties. A distinction is emphasized between the part of the billet directly affected, or as the authors style it, “explored,” by the mandrel, and the part which is unexplored, and the smaller the unexplored section is in relation to the full section of the solid billet, the fewer are the defects on the inside of the pierced tube. This leads to the conclusion that for the production of a given size tube, as small a solid billet as possible should be used. This is proved by a formula given for power consumption in piercing. The design of piercing mills is next considered from the same point of view and the most recent developments in the use of expanding mills are described.