Background: Light transmission aggregometry (LTA) is the most common method used in clinical and research laboratories to assess platelet function. However, the method has never been standardized. Objectives: As the first step towards development of methodological guidelines, the Platelet Physiology Subcommittee of the Scientific and Standardization Committee of the International Society on Thrombosis and Haemostasis (ISTH) undertook a large, detailed, global survey of LTA practices. Methods: Members of ISTH and of External Quality Assurance in Thrombosis and Haemostasis organizations were invited to complete a 129 item, online questionnaire. Results were analyzed anonymously to participant identities. Results: The online supplement for this article (http://www.isth.org/Publications/OfficialCommunications/PlateletPhysiology/LightTransmissionAggregometry/tabid/201/Default.aspx) contains the full details of the study findings. 359 (244 clinical, 115 research) laboratories from 48 countries participated in the survey. LTA was widely used to assess inherited or acquired bleeding disorders. Common practices were identified in sample collection, processing and analysis and although some are generally considered acceptable, others are not ideal. The agonist concentrations used for LTA varied, and many laboratories used ADP, collagen, epinephrine and Ristocetin, at more than one concentration, in addition to arachidonic acid. The parameters commonly used to assess LTA responses were maximal amplitude or % aggregation, which was considered particularly important, in addition to the presence of a ‘secondary wave’, deaggregation, shape change and a measure of the lag phase. However, many laboratories did not have appropriate reference intervals. Conclusions: This is the largest and most detailed survey of LTA practices ever undertaken. It shows a very high variability in LTA practices worldwide, and, as a consequence, methodological standardization is necessary. The information gathered in this survey will be helpful in the development of ISTH methodological guidelines for LTA.