The Bayley Scales of Infant and Toddler Development-Third Edition (Bayley-III) is a revision of the frequently used and well-known Bayley Scales of Infant Development-Second Edition (BSID-II; Bayley, 1993). Like its prior editions, the Bayley-III is an individually administered instrument designed to measure the developmental functioning of infants and toddlers. Other specific purposes of the Bayley-III are to identify possible developmental delay, inform professionals about specific areas of strength or weakness when planning a comprehensive intervention, and provide a method of monitoring a child's developmental progress. The Bayley-III is appropriate for administration to children between the ages of 1 month and 42 months (although norms extend downward to age 16 days). The revision of the Bayley was specifically driven by eight goals: (a) update the normative data, (b) develop additional scales to fulfill requirements by federal (i.e., the Individuals with Disabilities Education Improvement Act of 2004) and state laws regarding the five major areas of development for early childhood assessment from birth through 3 years of age, (c) strengthen the instrument's psychometric properties, (d) improve the treatment utility of the instrument, (e) simplify administration procedures, (f) update item administration, (g) update administration materials, and (h) maintain the qualities of previous Bayley editions (Bayley, 2006b). (author abstract)