The Bayley Scales of Infant and Toddler Development (Bayley-III is the current version) is a standard series of measurements originally developed by psychologist Nancy Bayley used primarily to assess the development of infants and toddlers, ages 1–42 months.This measure consists of a series of developmental play tasks and takes between 45 – 60 minutes to administer and derives a developmental quotient (DQ) rather than an intelligence quotient (IQ).

Raw scores of successfully completed items are converted to scale scores and to composite scores. These scores are used to determine the child’s performance compared with norms taken from typically developing children of their age (in months).

The most recent edition, the Bayley-III has three main subtests; the Cognitive Scale, which includes items such as attention to familiar and unfamiliar objects, looking for a fallen object, and pretend play, the Language Scale, which taps understanding and expression of language, for example, recognition of objects and people, following directions, and naming objects and pictures, and the Motor Scale, which assesses gross and fine motor skills such as grasping, sitting, stacking blocks, and climbing stairs.

There are two additional Bayley-II Scales depend on parental report, including the Social-Emotional scale, which asks caregivers about such behaviours as ease of calming, social responsiveness, and imitation play, and the Adaptive Behaviour scale which asks about adaptions to the demands of daily life, including communication, self-control, following rules, and getting along with others.

The Bayley-III Cognitive and Language scales are good predictors of preschool mental test performance.These scores are largely used for screening, helping to identify the need for further observation and intervention, as infants who score very low are at risk for future developmental problems.