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The pediatric neurologist is regularly asked to evaluate a hypotonic patient. This consultation request usually occurs in 2 different situations; the first is in the newborn period when the neurologist is asked to evaluate the "floppy infant," and the second is in the latter half of the first year of life and is usually accompanied by concern about the developmental progress of the infant and, in particular, the motor development of the infant. In this article, I will try to outline the factors related to the production of muscle tone in infants and children. The elements of the clinical evaluation of the hypotonic child including those clinical tests most helpful in the measurement of tone will be reviewed. A scheme for localizing the origin of the disturbance in muscle tone is presented, many of the known causes of the tone abnormalities are reviewed, and a rational approach to the diagnostic evaluation of these children is offered.