The control of particle morphology is an essential part of producing high quality latex products for applications in coatings, adhesives, impact modifiers, and medical diagnostics, among others. A great variety of formulation and process variables are available to manipulate the particle structure and many different morphologies have been created. Techniques to characterize these morphologies are varied, but electron microscopy of both whole and sectioned particles is the most common one used. Atomic force microscopy is gaining in utility and often two or more characterization methods are simultaneously used to gain clarity of interpretation. A great deal of basic understanding of the factors controlling the morphology has been achieved by applying equilibrium thermodynamics to phase separated particles in aqueous media. Interfacial tensions at the polymer-water interface and at the polymer-polymer interface, along with crosslinking density are found to be the dominant factors controlling the equilibrium morphology. In turn there is a significant number of formulation variables which determine the interfacial tensions and the crosslinking density. Successful models have been developed and applied to a number of different polymer systems. Much less progress has been made in understanding the development of non-equilibrium morphologies where the possible number of particle structures is essentially infinite. Here, characterization techniques become somewhat less precise in identifying exact structure and further work is needed to advance this capability. In the dynamic reaction environment of the latex process the morphology develops within very viscous phases and is a result of competitive reaction and diffusion processes. Some progress has been made to quantitatively describe these phenomena, but more work is needed. This is even more evident when extension to carboxylic and hybrid (e.g. polyurethane/acrylic) latices is desired.