A closure assembly for a container, the closure assembly comprising: a neck insert formed from a plastics material and comprising a. a through-going passageway, b. an annular base flange sealingly securable about an opening in the container so as to provide sealed fluid communication between the passageway and the opening in the container, and c. an open end through which fluid can be dispensed from the container; and a closure cap or plug removably securable to the neck insert so as to provide an openable fluid-tight seal across the open end; the closure cap or plug having a tamper-indicating anchor piece formed from a plastics material and attached to the closure cap or plug by a frangible connection, an edge part of the tamper-indicating anchor piece extending adjacent to a peripheral region of the annular base flange, whereby the closure assembly can be insert molded into the opening of the container, with material of the container sealingly molded around the peripheral region of the annular base flange and around the edge part of the tamper-indicating anchor piece; characterized in that an ultrasonic energy-directing protrusion spans between adjacent surfaces of the tamper-indicating anchor piece and of the annular base flange, and an annular, ultrasonic energy-directing ridge (58a, 58b) extends from a lower surface of the annular base flange, outboard of the through-going passageway, whereby as an alternative to securing by insert-molding, the tamper-indicating anchor piece is securable by ultrasonic welding to the annular base flange and the annular base flange is securable by ultrasonic welding to an outer wall surface of the container surrounding the opening in the container, so as to provide the sealed fluid communication between the passageway and the opening in the container. a neck insert formed from a plastics material and comprising a. a through-going passageway, b. an annular base flange sealingly securable about an opening in the container so as to provide sealed fluid communication between the passageway and the opening in the container, and c. an open end through which fluid can be dispensed from the container; and a closure cap or plug removably securable to the neck insert so as to provide an openable fluid-tight seal across the open end; the closure cap or plug having a tamper-indicating anchor piece formed from a plastics material and attached to the closure cap or plug by a frangible connection, an edge part of the tamper-indicating anchor piece extending adjacent to a peripheral region of the annular base flange, whereby the closure assembly can be insert molded into the opening of the container, with material of the container sealingly molded around the peripheral region of the annular base flange and around the edge part of the tamper-indicating anchor piece; characterized in that an ultrasonic energy-directing protrusion spans between adjacent surfaces of the tamper-indicating anchor piece and of the annular base flange, and an annular, ultrasonic energy-directing ridge (58a, 58b) extends from a lower surface of the annular base flange, outboard of the through-going passageway, whereby as an alternative to securing by insert-molding, the tamper-indicating anchor piece is securable by ultrasonic welding to the annular base flange and the annular base flange is securable by ultrasonic welding to an outer wall surface of the container surrounding the opening in the container, so as to provide the sealed fluid communication between the passageway and the opening in the container.