A safety pen needle assembly comprising: a hub having features for mounting onto a medical injector; a needle carrier; a needle fixed to said needle carrier, said needle including proximal and distal ends (24, 22), said distal end formed for insertion into a patient; a shield; first engageable locking means (38; 40, 42, 46, 48) for fixing said shield to said needle carrier; second engageable locking means (52; 54, 56) for fixing said needle carrier to said hub; wherein the shield is provided with a detent for snap engagement with a recess formed in the needle carrier; wherein a channel is provided in the needle carrier formed to accommodate sliding movement of the detent to form the first engageable locking means, wherein, in an initial state, said shield is moveable relative to said needle carrier, wherein, upon a predetermined extent of distal movement of said shield relative to said hub, said first locking means (38; 40, 42, 46, 48) is engaged such that said shield becomes fixed to said needle carrier covering the distal end of said needle, wherein, upon further distal movement of said shield relative to said hub beyond said predetermined extent of distal movement, said shield and said needle carrier move in concert together relative to said hub causing said proximal end of said needle to move distally relative to said hub to a retracted state and bringing said second locking means (52; 54, 56) into engagement, characterized in that said needle carrier further includes a structure to limit inadvertent activation of the shielding prior to injection by releasably preventing axial movement of the shield relative to said needle carrier, wherein said structure is configured such that rotational movement of said shield relative to said needle carrier allows axial movement of the shield relative to the needle carrier, said structure comprising a storage channel defined in the needle carrier disposed transversely to the channel, wherein a storage recess is provided along the storage channel having adjacent thereto a storage ramped surface, and wherein prior to use the detent is seated in the storage recess out of longitudinal alignment with the channel. a hub having features for mounting onto a medical injector; a needle carrier; a needle fixed to said needle carrier, said needle including proximal and distal ends (24, 22), said distal end formed for insertion into a patient; a shield; first engageable locking means (38; 40, 42, 46, 48) for fixing said shield to said needle carrier; second engageable locking means (52; 54, 56) for fixing said needle carrier to said hub; wherein the shield is provided with a detent for snap engagement with a recess formed in the needle carrier; wherein a channel is provided in the needle carrier formed to accommodate sliding movement of the detent to form the first engageable locking means, wherein, in an initial state, said shield is moveable relative to said needle carrier, first engageable locking means (38; 40, 42, 46, 48) for fixing said shield to said needle carrier; second engageable locking means (52; 54, 56) for fixing said needle carrier to said hub; wherein the shield is provided with a detent for snap engagement with a recess formed in the needle carrier; wherein a channel is provided in the needle carrier formed to accommodate sliding movement of the detent to form the first engageable locking means, wherein, in an initial state, said shield is moveable relative to said needle carrier, wherein, upon a predetermined extent of distal movement of said shield relative to said hub, said first locking means (38; 40, 42, 46, 48) is engaged such that said shield becomes fixed to said needle carrier covering the distal end of said needle, wherein, upon further distal movement of said shield relative to said hub beyond said predetermined extent of distal movement, said shield and said needle carrier move in concert together relative to said hub causing said proximal end of said needle to move distally relative to said hub to a retracted state and bringing said second locking means (52; 54, 56) into engagement, characterized in that said needle carrier further includes a structure to limit inadvertent activation of the shielding prior to injection by releasably preventing axial movement of the shield relative to said needle carrier, wherein said structure is configured such that rotational movement of said shield relative to said needle carrier allows axial movement of the shield relative to the needle carrier, said structure comprising a storage channel defined in the needle carrier disposed transversely to the channel, wherein a storage recess is provided along the storage channel having adjacent thereto a storage ramped surface, and wherein prior to use the detent is seated in the storage recess out of longitudinal alignment with the channel.