Observational (behavioral) scales of pain for children aged 3 to 18 years were systematically reviewed to identify those recommended as outcome measures in clinical trials. This review was commissioned by the Pediatric Initiative on Methods, Measurement, and Pain Assessment in Clinical Trials (www.immpact.org). In an extensive literature search, 20 observational pain scales were identified for review including behavior checklists, behavior rating scales, and global rating scales. These scales varied in their reliance on time sampling and inclusion of physiological items, facial and postural items, as well as their inclusion of multiple dimensions of assessment (e.g., pain and distress). Each measure was evaluated based on its reported psychometric properties and clinical utility. Scales were judged to be indicated for use in specific acute pain contexts rather than for general use. Two scales were recommended for assessing pain intensity associated with medical procedures and other brief painful events. Two scales were recommended for post-operative pain assessment, one for use in hospital and the other at home. Another scale was recommended for use in critical care. Finally, two scales were recommended for assessing pain-related distress or fear. No observational measures were recommended for assessing chronic or recurrent pain because the overt behavioral signs of chronic pain tend to habituate or dissipate as time passes, making them difficult to observe reliably. In conclusion, no single observational measure is broadly recommended for pain assessment across all contexts. Directions for further research and scale development are offered.