The global increase in myopia prevalence and associated secondary complications necessitates adequate monitoring of myopia progression. This review highlights the advantages and disadvantages of the most commonly used target parameters for assessing myopia progression: refractive error and axial length. Although refractive error is essential for the diagnosis and optical correction of myopia, axial length proves to be a more reliable and clinically relevant parameter for monitoring progression and guiding preventive interventions. Axial length correlates more strongly with the risk of ocular complications and is less susceptible to diurnal variations and interventions that alter the eye's refractive power. Assessing myopia progression under preventive measures requires consideration of age-dependent physiological changes in axial length. Standardized comparison parameters, such as the cumulative absolute reduction in axial elongation (CARE) can facilitate the objective evaluation of the efficacy of the intervention. Regular monitoring of refraction and axial length should also be performed in young adults with risk factors, as myopia onset and progression can occur even at this age.