New pre-school UK charts have been produced incorporating the new World Health Organization growth standards based on healthy breastfed infants. This paper describes the process by which the charts and evidence-based instructions were designed and evaluated, and what it revealed about professional understanding of charts and growth monitoring. A multidisciplinary expert group drew on existing literature, new data analyses and parent focus groups as well as two series of chart-plotting workshops for health staff. The first series explored possible design features and general chart understanding. The second evaluated an advanced prototype with instructions, using plotting and interpretation of three separate scenarios on the old charts, compared with the new charts. The first plotting workshops (46 participants) allowed decisions to be made about the exact chart format, but it also revealed widespread confusion about use of adjustment for gestation and the plotting of birthweight. In the second series (78 participants), high levels of plotting inaccuracy were identified on both chart formats, with 64% of respondents making at least one major mistake. Significant neonatal weight loss was poorly recognized. While most participants recognized abnormal and normal growth patterns, 13-20% did not. Many respondents had never received any formal training in chart use. Growth charts are complex clinical tools that are, at present, poorly understood and inconsistently used. The importance of clear guidelines and formal training has now been recognized and translated into supporting educational materials (free to download at http://www.growthcharts.rcpch.ac.uk).