A method of cementing an oil or gas well for abandonment comprises cutting the production tubing 6 above the production packer 8, setting a tubing plug 9 in the tubing and then cutting the tubing. A bridge plug may be installed at the level of the cut, which spans the full diameter of the well casing 3. An agitator assembly 20 comprising an agitator 24, packer 21 and burst sub 23, with a running tool fitted to the top, is run down the tubing on wire line. Cement is then injected into the tubing 6, which flows through the agitator assembly and causes the tubing to vibrate. The cement fills the tubing but also flows back up the A annulus between the casing and production tubing. Vibration of the tubing assists the formation of a good quality cement plug extending all around the annulus over a substantial length of the well.