For the past 60 years, light transmission aggregometry (LTA) has been a crucial tool in specialized laboratories to identify defects in essential platelet components in patients with bleeding disorders. Significant advances have been made to standardize the implementation and interpretation of this labor-intensive technique. While LTA is highly effective in diagnosing both hereditary and acquired platelet disorders, it does not fully capture the heterogeneity of platelet activation in vivo, in which various components of platelet aggregates are integrated over time and space, influenced by fluid dynamics and platelet activators. This chapter discusses the characteristics of traditional LTA and the advances in platelet aggregation testing. It describes a significant shift from conventional methods to automated and miniaturized technologies, which have the potential to enhance our understanding of the significance of platelet aggregation in various clinical contexts.