The WISC-IV is likely to be in common use for the assessment of children with low intellectual ability for the next 10-12 years. There are several concerns about its uses with these children. Some children may not understand the instructions on some subtests, notably for letter-number sequencing. There may be an unacknowledged floor effect that will result in an overestimate of intellectual ability. The percentile ratings given in the manual are inaccurate for percentiles below 1%. Evidence is presented that the WISC-IV may systematically give lower IQ scores than the WAIS-III when assessing low IQ.