The ultimate aim of this PhD research work is the formalisation and validation of a novel method suitable for designing notched metallic components against proportional/non-proportional constant/variable amplitude multiaxial Fatigue. The above method will take as a starting point the assumption that the plane experiencing the maximum shear strain amplitude is coincident with the micro crack initiation plane. In order to correctly account for the presence of both non-zero mean stresses and non-zero out-of-phase angles, the degree of multiaxiality of the stress state damaging crack initiation sites will be evaluated in terms of ratio between maximum normal stress and shear stress amplitude relative to the critical plane. Such a ratio will then be used to define non-conventional Manson-Coffin curves, their calibration being based on the combined use of the strain-life curves generated under uniaxial and torsional fully-reversed constant amplitude fatigue loading. Subsequently, in order to properly take into account the detrimental effect of notches, the above novel elastoplastic multiaxial fatigue criterion will be reformulated in terms of the so-called Theory of Critical Distances. Finally, the accuracy and reliability of the design method aimed to be devised through the present PhD research project will be checked by performing a systematic experimental investigation at the structural integrity laboratory of the University of Sheffield.