Providing reference values in the form of percentiles for the assessment of paediatric growth is of great practical importance. Extreme centiles are particularly important in the diagnostic process. On the other hand, their statistical variability is inherently large. When measurements approximate to a normal distribution -as for height – centiles can be obtained more accurately. Otherwise, empirical centiles and/or more refined methods are needed. To exploit the age-dependent nature of centiles, smoothing methods are recommended. Accounting for midparental height as a covariate in the calculation of centiles greatly improves their validity due to an increased sensitivity and specificity.