SE Asia is renowned as a region of complex plate tectonic interactions, both in the present day and throughout its Mesozoic and Cenozoic history. The study of subduction processes in SE Asia has been instrumental in our understanding of how complex subduction systems develop and evolve, including understanding double subduction zones, areas of subduction polarity reversal, and the interaction of subduction and strike-slip systems. This complexity in subduction style makes SE Asia an ideal natural laboratory for studying forearc development in a range of subduction styles across a relatively small area still exposed in the rock record. Here we present a recently studied example of forearc development in a Mesozoic double subduction zone exposed on Natuna Island in the South China Sea, as well as highlighting two other examples of forearc development in SE Asia. These include a Cenozoic subduction polarity reversal event and transform plate boundary in western New Guinea and forearc sedimentation along the Sunda Trench. All three scenarios chronicle histories of forearc accretion, either of deep-water cherts or continental-derived turbidites, whilst also recording the impacts of case-specific tectonic processes (such as ophiolite obduction, arc-continent collision, or strike-slip movement) that have fundamentally impacted their respective forearcs. Comparing these contrasting examples shows that studying forearc development of these complex subduction systems (including their structural styles, geochemistry and timing of associated magmatism, and sedimentation) in SE Asia can be a powerful tool for improving understanding of forearc evolution in other ancient and complex subduction systems.