A therapeutic fluid infusion system for delivering a therapeutic fluid to a human body, the system comprising: a first assembly configured as a needle unit comprising: a cradle having a flat bottom surface configured for adhesion to a cutaneous region of the human body, a well, a cannula, and a self- sealable septum, wherein a distal portion of the cannula is configured for subcutaneous placement within the human body and wherein the self- sealable septum separates a proximal portion of the cannula from an external environment; and a second assembly configured as a patch unit and configured for removable attachment to the first assembly, the second assembly comprising: energy supply means, electronic components, a driving mechanism, a pumping mechanism, a reservoir for containing a therapeutic fluid, and an exit port with a connecting lumen configured to penetrate the self- sealable septum in order to bring the second assembly in fluid communication with the first assembly, and a delivery tube connecting the reservoir to the exit port; wherein the first and second assemblies (10, 20) are connectable by a sliding movement of the second assembly relative to the first assembly in a direction parallel to the bottom surface of the cradle, and characterized in that the second assembly comprises: a disposable part comprising the delivery tube, the reservoir and the exit port with the connecting lumen; and a reusable part comprising at least one of: the electronic components; the driving and pumping mechanisms; and other relatively expensive components. a first assembly configured as a needle unit comprising: a cradle having a flat bottom surface configured for adhesion to a cutaneous region of the human body, a well, a cannula, and a self- sealable septum, wherein a distal portion of the cannula is configured for subcutaneous placement within the human body and wherein the self- sealable septum separates a proximal portion of the cannula from an external environment; and a cradle having a flat bottom surface configured for adhesion to a cutaneous region of the human body, a well, a cannula, and a self- sealable septum, wherein a distal portion of the cannula is configured for subcutaneous placement within the human body and wherein the self- sealable septum separates a proximal portion of the cannula from an external environment; a second assembly configured as a patch unit and configured for removable attachment to the first assembly, the second assembly comprising: energy supply means, electronic components, a driving mechanism, a pumping mechanism, a reservoir for containing a therapeutic fluid, and an exit port with a connecting lumen configured to penetrate the self- sealable septum in order to bring the second assembly in fluid communication with the first assembly, and a delivery tube connecting the reservoir to the exit port; energy supply means, electronic components, a driving mechanism, a pumping mechanism, a reservoir for containing a therapeutic fluid, and an exit port with a connecting lumen configured to penetrate the self- sealable septum in order to bring the second assembly in fluid communication with the first assembly, and a delivery tube connecting the reservoir to the exit port; wherein the first and second assemblies (10, 20) are connectable by a sliding movement of the second assembly relative to the first assembly in a direction parallel to the bottom surface of the cradle, and characterized in that the second assembly comprises: a disposable part comprising the delivery tube, the reservoir and the exit port with the connecting lumen; and a reusable part comprising at least one of: the electronic components; the driving and pumping mechanisms; and other relatively expensive components. a disposable part comprising the delivery tube, the reservoir and the exit port with the connecting lumen; and a reusable part comprising at least one of: the electronic components; the driving and pumping mechanisms; and other relatively expensive components.