Speech intelligibility is a fundamental feature of spoken communication, involving the production of an acoustic signal by a speaker and the decoding of that signal by a listener. Intelligibility impairment is an issue across the lifespan, for a range of clinical populations that frequent the speech-language pathologist's caseload. For children, an understanding of developmental trajectories and the range of typical variability by age is critical for identifying those who have delays and disorders. Clinically, intelligibility measures provide an important metric that can be used for treatment decision-making. Operationally, there are two main approaches to measuring intelligibility: objective measures and subjective measures. When listeners encounter speech that is difficult to understand, they rapidly adapt their perceptual systems, mapping the non-canonical acoustic cues onto linguistic categories stored in memory. This is known as perceptual learning.