The use of fracture mechanics based methodologies to assess the influence of defects on the high cycle fatigue resistance of mechanical components has become increasingly important for industrial applications and design. This presentation introduces the most commonly used fracture mechanics methods, emphasizing the assumptions on which they are based and analyzing their potential in predicting the influence of the main geometrical, mechanical and microstructural variables involved in the definition of the fatigue behaviour of the components. Applications examples on different configurations with small defects are presented. Special attention is paid to the example of welded joints, a complex configuration with defects, stress concentrations, residual stresses, microstructure distributions, etc. Finally, the potential of the fracture mechanics methodologies is analyzed when proposing simplifications suitable for use in the development of design documents for engineering applications.