A filling unit for filling containing elements of single-use capsules with a dose of product for extraction or infusion beverages, comprising: - a line for transporting the containing elements extending along a first movement path and provided with a plurality of supporting seats for the containing elements arranged in succession along the first movement path; - a station for filling the above-mentioned containing elements with a dose of product; characterised in that the filling station comprises: - at least one first containing seat designed to receive a dose of product and movable along a second movement path; - a substation for forming the dose inside the at least one first containing seat, provided with a device for releasing a predetermined quantity of product forming the dose inside the at least one first containing seat; - at least one second containing seat designed to receive the dose of product from the at least one first containing seat and movable along a third movement path; - a substation for transferring the dose of product from the at least one first containing seat to the at least one second containing seat; - devices for moving the at least one first containing seat between the forming substation and the transfer substation and vice versa; - a substation for releasing the dose of product from the at least one second containing seat to a containing element transported by the transport line; - further devices for moving the at least one second containing seat, designed to move the second containing seat along the third movement path from the transfer substation to the release substation and vice versa, characterized by the transfer substation and the release substation being positioned at a predetermined distance from one another along the third movement path, the third movement path being parallel to the first movement path of the transport line at the release substation. - a line for transporting the containing elements extending along a first movement path and provided with a plurality of supporting seats for the containing elements arranged in succession along the first movement path; - a station for filling the above-mentioned containing elements with a dose of product; - at least one first containing seat designed to receive a dose of product and movable along a second movement path; - a substation for forming the dose inside the at least one first containing seat, provided with a device for releasing a predetermined quantity of product forming the dose inside the at least one first containing seat; - at least one second containing seat designed to receive the dose of product from the at least one first containing seat and movable along a third movement path; - a substation for transferring the dose of product from the at least one first containing seat to the at least one second containing seat; - devices for moving the at least one first containing seat between the forming substation and the transfer substation and vice versa; - a substation for releasing the dose of product from the at least one second containing seat to a containing element transported by the transport line; - further devices for moving the at least one second containing seat, designed to move the second containing seat along the third movement path from the transfer substation to the release substation and vice versa, characterized by the transfer substation and the release substation being positioned at a predetermined distance from one another along the third movement path, the third movement path being parallel to the first movement path of the transport line at the release substation.