The invention provides in one aspect a drug delivery device comprising a drug container assembly containing a drug, and a plunger positioned within the container assembly, the drug container assembly having an outlet for dispensing the drug, wherein, in an initial position, the outlet is sealed； a drive mechanism comprising a first stored energy source operable to apply pressure on the plunger or the drug container to pressurise the drug, and a first release mechanism operable to unseal the outlet after the drug has been pressurised. In another aspect, there is provided a drug delivery device comprising a housing, including an external housing portion configured to be held in use and a drug containing portion containing a drug, a needle assembly comprising a needle fixed to a needle hub, the needle hub configured to move relative to the drug containing portion from an initial position within the housing to an insertion position in which the needle extends beyond the housing, a first stored energy source configured to move the needle hub from the initial position to the insertion position, and a plunger within the housing and configured to move relative to the drug containing portion to eject the drug through the needle when the needle is in the insertion position.
