An applicator for applying a microneedle device to a skin surface, the applicator comprising: a housing having a first surface configured to be positioned toward the skin surface, the housing comprising a bore that extends through the first surface; a deformable sheet positioned to extend across the bore of the housing, spaced a distance from the first surface of the housing, the deformable sheet having a first side oriented toward the first surface of the housing and a second side opposite the first side; a microneedle device coupled to the first side of the deformable sheet, such that the microneedle device is oriented toward the first surface of the housing; a plunger dimensioned to be at least partially received in the bore of the housing adjacent the second side of the deformable sheet and located opposite the deformable sheet from the microneedle device, the plunger being movable relative to the housing between a first position and a second position to move the microneedle device, respectively, between a first position (M 1) in which the microneedle device is recessed relative to the first surface of the housing, and a second position (M 2) in which at least a portion of the microneedle device extends beyond the first surface of the housing; and a locking mechanism configured to lock the microneedle device in the second position, wherein: the first position (P 1) of the plunger is a first unlocked position in which the locking mechanism is not activated, the second position (P 2) of the plunger is a second locked position in which the locking mechanism is activated when the plunger is moved into the second locked position, and the plunger is further movable to a third unlocked position (P 3) located intermediately of the first position (P 1) of the plunger and the second position (P 2) of the plunger in which at least a portion of the microneedle device extends beyond the first surface of the housing but in which the locking mechanism is not activated. a housing having a first surface configured to be positioned toward the skin surface, the housing comprising a bore that extends through the first surface; a deformable sheet positioned to extend across the bore of the housing, spaced a distance from the first surface of the housing, the deformable sheet having a first side oriented toward the first surface of the housing and a second side opposite the first side; a microneedle device coupled to the first side of the deformable sheet, such that the microneedle device is oriented toward the first surface of the housing; a plunger dimensioned to be at least partially received in the bore of the housing adjacent the second side of the deformable sheet and located opposite the deformable sheet from the microneedle device, the plunger being movable relative to the housing between a first position and a second position to move the microneedle device, respectively, between a first position (M 1) in which the microneedle device is recessed relative to the first surface of the housing, and a second position (M 2) in which at least a portion of the microneedle device extends beyond the first surface of the housing; and a first position (M 1) in which the microneedle device is recessed relative to the first surface of the housing, and a second position (M 2) in which at least a portion of the microneedle device extends beyond the first surface of the housing; and a locking mechanism configured to lock the microneedle device in the second position, wherein: the first position (P 1) of the plunger is a first unlocked position in which the locking mechanism is not activated, the second position (P 2) of the plunger is a second locked position in which the locking mechanism is activated when the plunger is moved into the second locked position, and the plunger is further movable to a third unlocked position (P 3) located intermediately of the first position (P 1) of the plunger and the second position (P 2) of the plunger in which at least a portion of the microneedle device extends beyond the first surface of the housing but in which the locking mechanism is not activated. the first position (P 1) of the plunger is a first unlocked position in which the locking mechanism is not activated, the second position (P 2) of the plunger is a second locked position in which the locking mechanism is activated when the plunger is moved into the second locked position, and the plunger is further movable to a third unlocked position (P 3) located intermediately of the first position (P 1) of the plunger and the second position (P 2) of the plunger in which at least a portion of the microneedle device extends beyond the first surface of the housing but in which the locking mechanism is not activated.