The broken stick model describes a set of individual curves by a linear mixed model using a second-order linear B-spline. The main use of the model is to align irregularly observed data to a user-specified grid of break ages. All fitting can done in the Z-score scale, so non-linearity and irregular data can be treated as separate problems. This package contains functions for fitting a broken stick model to data, for predicting broken stick curves in new data, and for plotting the broken stick estimates. For additional documentation on background, methodology and applications of the broken stick model see < https://stefvanbuuren.name/publications/2020_Brokenstick_JSS_manuscript.pdf>.