A completion system comprises a christmas tree (10) mounted on a wellhead housing (11), a tubing hanger (12) landed in the tree or wellhead housing, the wellhead housing (11) being mounted on a casing string (100) and a tubing string (14) being suspended from the tubing hanger within the casing string; wherein, in use, the annulus defined between the tubing (14) and the casing (100) serves as a production bore. A second tubing string (98) is expanded into sealing engagement with the casing string (100) over at least a portion of their lengths. The annulus normally used to provide well service functions is thus eliminated. Well servicing is instead provided via the tubing string (14), which may be coiled tubing.