A system for delivering a fluid, the system comprising: an insertable reservoir system comprising: a reservoir body having an internal volume for the fluid to be delivered; a plunger arm operatively connected to a plunger head arranged for axial movement within the reservoir body to expel the fluid; a casing configured to support the plunger arm and to envelop at least a portion of the plunger arm and having an opening to expose an engagement portion of the plunger arm; and a front face of the reservoir body; and a housing including: a drive member positioned to engage the engagement portion of the plunger arm through the opening when the reservoir system is in the housing, to axially drive the plunger arm and plunger head; a datum point inside the housing to position against the front face when the insertable reservoir system is positioned in the housing; and a retention structure comprising a rigid member mounted for movement from an extended position to an retracted position against the force of a bias member; wherein the insertable reservoir system has a flange positioned to engage a face of the retention structure as the reservoir system is inserted into the housing, to compress the bias member towards the retracted position, the bias member being configured to urge the rigid member towards the extended position such that the rigid member provides a force against the casing in a first direction to urge the plunger arm against the drive member, and in a second direction to urge the front face of the insertable reservoir system against the datum point, the first direction being perpendicular to the second direction. an insertable reservoir system comprising: a reservoir body having an internal volume for the fluid to be delivered; a plunger arm operatively connected to a plunger head arranged for axial movement within the reservoir body to expel the fluid; a casing configured to support the plunger arm and to envelop at least a portion of the plunger arm and having an opening to expose an engagement portion of the plunger arm; and a front face of the reservoir body; and a reservoir body having an internal volume for the fluid to be delivered; a plunger arm operatively connected to a plunger head arranged for axial movement within the reservoir body to expel the fluid; a casing configured to support the plunger arm and to envelop at least a portion of the plunger arm and having an opening to expose an engagement portion of the plunger arm; and a front face of the reservoir body; and a housing including: a drive member positioned to engage the engagement portion of the plunger arm through the opening when the reservoir system is in the housing, to axially drive the plunger arm and plunger head; a datum point inside the housing to position against the front face when the insertable reservoir system is positioned in the housing; and a retention structure comprising a rigid member mounted for movement from an extended position to an retracted position against the force of a bias member; wherein the insertable reservoir system has a flange positioned to engage a face of the retention structure as the reservoir system is inserted into the housing, to compress the bias member towards the retracted position, the bias member being configured to urge the rigid member towards the extended position such that the rigid member provides a force against the casing in a first direction to urge the plunger arm against the drive member, and in a second direction to urge the front face of the insertable reservoir system against the datum point, the first direction being perpendicular to the second direction. a drive member positioned to engage the engagement portion of the plunger arm through the opening when the reservoir system is in the housing, to axially drive the plunger arm and plunger head; a datum point inside the housing to position against the front face when the insertable reservoir system is positioned in the housing; and a retention structure comprising a rigid member mounted for movement from an extended position to an retracted position against the force of a bias member; wherein the insertable reservoir system has a flange positioned to engage a face of the retention structure as the reservoir system is inserted into the housing, to compress the bias member towards the retracted position, the bias member being configured to urge the rigid member towards the extended position such that the rigid member provides a force against the casing in a first direction to urge the plunger arm against the drive member, and in a second direction to urge the front face of the insertable reservoir system against the datum point, the first direction being perpendicular to the second direction.