Eighteen pressure vessel heads having nine different geometries and made from two different steels have been subjected to monotonically increasing external pressure until collapse occurred.Deflection of the crown and strains in the most highly strained regions of the concave side of the heads were recorded as functions of pressure. All heads underwent permanent deformations. Four modes of failure were observed: (i) symmetrical buckling of the crown, (ii) asymmetrical buckling in the knuckle and transition region, (iii) asymmetrical buckling of the cylindrical wall, and (iv) progressive deformation without buckling.These test results were compared with predictions by formulas proposed by Slember and Washington, the ASME Code, and by the first yield criteria.Four new formulas were developed and compared with the test results and selected data from Bart.