Achilles tendon orthosis designated for treating an Achilles tendon of a foot, - the Achilles tendon having an orientation Y, - the foot in a cross section taken perpendicular to the orientation Y and taken through the Achilles tendon having an outer contour, - the outsides of the skin parts of the foot covering the Achilles tendon having a distance X, the Achilles tendon orthosis comprising a) a compression structure extending in a compression structure plane having an orientation perpendicular to the orientation Y and b) two compression elements (13, 14) forming elongate opposing contact surfaces (15, 16) having an orientation parallel to the orientation Y, the compression elements (13, 14) being supported by the compression structure and being designated for contacting the foot in contact areas, c) a biasing mechanism which allows a change of the configuration of the Achilles tendon orthosis between ca) a non-biasing state wherein the contact surfaces (15, 16) have a distance being larger than or equal to X so that when applying the Achilles tendon orthosis the compression elements (13, 14) do not apply a compression force to the contact areas and cb) a biasing state wherein the contact surfaces (15, 16) have a distance being smaller than X when no compression counterforces are applied to the contact surface (15, 16) of the compression elements (13, 14) so that when applying the Achilles tendon orthosis the compression elements (13, 14) apply a compression force to the contact areas, d) the Achilles tendon orthosis being dimensioned, configured and designed such that when applying the Achilles tendon orthosis to a foot the contact surfaces (15, 16) of the compression elements (13, 14) contact the foot at a location anterior to the mid-portion Achilles tendon and posterior to the lower leg and/or ankle joint and characterized in that e) the compression structure comprises supporting elements (9, 10) having an inherent rigidity, the supporting elements (9, 10) support the compression elements (13, 14) and by compression counterforces acting upon the contact surfaces (15, 16) of the compression elements (13, 14) the supporting elements (9, 10) are biased by a bending moment. - the Achilles tendon having an orientation Y, - the foot in a cross section taken perpendicular to the orientation Y and taken through the Achilles tendon having an outer contour, - the outsides of the skin parts of the foot covering the Achilles tendon having a distance X, a) a compression structure extending in a compression structure plane having an orientation perpendicular to the orientation Y and b) two compression elements (13, 14) forming elongate opposing contact surfaces (15, 16) having an orientation parallel to the orientation Y, the compression elements (13, 14) being supported by the compression structure and being designated for contacting the foot in contact areas, c) a biasing mechanism which allows a change of the configuration of the Achilles tendon orthosis between ca) a non-biasing state wherein the contact surfaces (15, 16) have a distance being larger than or equal to X so that when applying the Achilles tendon orthosis the compression elements (13, 14) do not apply a compression force to the contact areas and cb) a biasing state wherein the contact surfaces (15, 16) have a distance being smaller than X when no compression counterforces are applied to the contact surface (15, 16) of the compression elements (13, 14) so that when applying the Achilles tendon orthosis the compression elements (13, 14) apply a compression force to the contact areas, ca) a non-biasing state wherein the contact surfaces (15, 16) have a distance being larger than or equal to X so that when applying the Achilles tendon orthosis the compression elements (13, 14) do not apply a compression force to the contact areas and cb) a biasing state wherein the contact surfaces (15, 16) have a distance being smaller than X when no compression counterforces are applied to the contact surface (15, 16) of the compression elements (13, 14) so that when applying the Achilles tendon orthosis the compression elements (13, 14) apply a compression force to the contact areas, d) the Achilles tendon orthosis being dimensioned, configured and designed such that when applying the Achilles tendon orthosis to a foot the contact surfaces (15, 16) of the compression elements (13, 14) contact the foot at a location anterior to the mid-portion Achilles tendon and posterior to the lower leg and/or ankle joint and