The assessment of platelet function by aggregation assays is helpful in the diagnosis of many platelet function disorders. Platelet aggregation responses to agonists can be assessed by light transmittance methods (using platelet-rich plasma and, less commonly, washed platelets) and by whole blood (impedance) methods. The interpretation of aggregation findings is a complex process, requiring careful consideration of the abnormal responses, potential preanalytical and analytical artifacts, and the conditions that could be responsible for the aggregation abnormalities. Platelet dense granule release, which accompanies aggregation responses to many agonists, can be measured by lumiaggregometry to improve the detection and subclassification of platelet function disorders. The proper assessment of platelet function disorders by aggregometry requires quality control of the test and the use of established, validated reference intervals, and proper test interpretation, ideally based on guidelines.