Piecewise growth models (PGMs) were used to estimate and model changes in the preliteracy skill development of kindergartners in a moderately sized school district in the Pacific Northwest. PGMs were applied to interrupted time-series (ITS) data that arose within the context of a response-to-intervention (RtI) instructional framework. During the kindergarten year, multiple literacy assessments were conducted and supplemental instruction was initiated with struggling readers to promote the attainment of literacy benchmark goals. The use of PGMs provided analytic flexibility to specify a discontinuous learning function and test a number of within- and between-group model parameters related to the evaluation of program efficacy. Results revealed a statistically significant linear increment in student learning following the onset of the intervention. However, absolute and relative gains in literacy performance were more modest during the second half of the academic year. Key issues in applying PGMs within the RtI framework, including the coding of time functions and the interpretation of model results, are addressed.