  A calculation of the pion mass and decay constant at NNNLO in two-flavour
chiral perturbation theory is presented. The results are cross-checked by using
both the exponential and square root parameterizations of the Goldstone matrix
field, as well as by comparing to the known leading log coefficients of the two
quantities. A small numerical study of the quark mass dependence is performed,
and for a physical quark mass there is good agreement with lower order results.
