SummaryFor some time it has been known that “complex” sound stimuli are more effective than pure tones in eliciting behavioural and other responses from infants aged 0–6 months. We have recently studied this issue in some detail, and confirm that stimulus complexity defined in terms of frequency bandwidth is of major importance in eliciting behavioural responses from infants aged 1 week, 6 weeks and 6 months. We have also found that the human voice is effective for 6 month old infants but apparently not for babies aged 1 and 6 weeks. These findings are modified by the baby's state of wakefulness, the effects of which are also considered. Comments are made on the implications of our data for clinical audiological assess