Electrochemiluminescence (ECL) is a surface-confined process, where light emission arises from short-lived intermediates generated near the electrode after an electrochemical stimulus. The spatial extent of the emitting region, known as the thickness of the ECL layer (TEL), is primarily governed by the diffusion behavior of these intermediates. In this chapter, the use of Electrochemiluminescence Microscopy (ECLM) is explored as a powerful technique for visualizing and understanding TEL formation, providing spatially resolved insight into ECL mechanisms. A variety of strategies to probe and control TEL are presented, including the use of advanced electrode architectures and tailored luminophores and coreactants. Modeling approaches, especially finite element simulations, are also discussed as essential tools to interpret complex spatial and kinetic behaviors in ECL systems. Through this framework, ECLM is shown not only as an imaging tool, but as a mechanistic probe that offers a deeper understanding of light generation processes and supports the development of improved ECL-based sensing platforms.