Fundamental concepts of functionality within polymer systems are restated, with particular reference to the phenol-formaldehyde (P-F) step-growth polymerization. The functionality of both reactants is governed by the stoichiometry. A distinction between the number of functional groups (potential functionality) and actual functionality is made. At the extent of reaction used in practical systems when P/F > 1 (i.e. novolacs) the actual functionality of formaldehyde will always be two, while the actual functionality of phenol will approach a maximum of two. When P/F < 1 (i.e. resoles), a more complex situation arises, with formaldehyde never achieving an actual functionality of two while phenol may now attain an actual functionality greater than two. The functionality value for phenol within novolac resins of 2.31 quoted in the literature is shown to be in error. Results from an accurate molecular weight analysis of several industrial resins and using pure novolac-like compounds as g.p.c. standards give rise to actual functionality values of phenol in the range 1.49-1.72. This in turn can be used to calculate P/F ratios which range from 1.21 to 1.34. These compare well with typical starting P/F ratios used during the synthesis of P-F resins.