Perspective on the paper by Olsen et al ( see 109 ) Failure to thrive (FTT) or weight faltering has been regarded as an important problem in infancy1 associated with a range of paediatric conditions from coeliac disease2 to sudden infant death syndrome.3 Growth monitoring, perhaps the most common child surveillance activity,4 aims, among other things, to identify faltering growth,5 and generates frequent paediatric referrals.6 However, as Olsen et al 7 show, the definition of FTT is open to dispute, raising questions about the value of the weight component of growth monitoring and the continued place of weight faltering as a useful marker of disease and impaired infant development. Despite its established place in the paediatric literature (a Medline search limited to all infants and humans only with “failure to thrive” as the keyword yields >2000 references), there is no consensus on its precise definition. Undernutrition is thought to underlie FTT,8 and a recent review of articles published between January 2003 and June 2004 and recent textbooks reported that solely anthropometric parameters are now universally used in its identification.9 However, there was no agreement on which growth parameters to use and whether to use attained values or velocities.9 Thus, it seems that static definitions of low attained weight continue to be used despite longstanding recognition of their limitations.10,11 Dynamic definitions that assess weight velocity and change over time are now regarded by most researchers as preferable to attained values. However, there is limited consensus as to which dynamic definitions to use. The most frequently used in practice are those in which FTT is defined as a fall through centile lines. For example, O’Brien et al 12 defined FTT as weight falling through two major centile lines on standard weight charts …