An isolation tool for use in a pipe comprises a seal unit, the seal unit being mounted to a launching assembly. The seal unit is adapted for location in a section of pipe, the seal unit adapted to be inserted into the pipe via a single opening in the pipe wall. The seal unit comprises seal elements for engaging the pipe wall to isolate a section of pipe. The seal elements are initially retained in a retracted configuration. Hydraulic pressure is applied to a cylinder, a piston located in the cylinder being translated into the seal unit, causing compression of the seal elements.