The dynamics of supercooled liquid and glassy systems are usually studied within the Lagrangian representation, in which the positions and velocities of distinguishable interacting particles are followed. Within this representation, however, it is difficult to define measures of spatial heterogeneities in the dynamics, as particles move in and out of any one given region within long enough times. It is also nontransparent how to make connections between the structural glass and the spin glass problems within the Lagrangian formulation. We propose an Eulerian formulation of supercooled liquids and glasses that allows for a simple connection between particle and spin systems, and that permits the study of dynamical heterogeneities within a fixed frame of reference similar to the one used for spin glasses. We apply this framework to the study of the dynamics of colloidal particle suspensions for packing fractions corresponding to the supercooled and glassy regimes, which are probed via confocal microscopy.dynamics ͉ structural T he phenomenology of structural and spin glasses has much in common: no static long-range order, aging relaxation, heterogeneous dynamics, and so on (for reviews, see refs. 1 and 2). Although a precise and unambiguous connection between these two problems is still lacking, the possibility that such relation exists dates back to the work by Kirkpatrick, Thirumalai,, who proposed a connection between structural glasses and fully connected p-spin disordered models. These mean-field spin models have a dynamic phase transition that mimics the glassy arrest at T g and a static phase transition at a lower temperature T s that realizes the Kauzmann entropy crisis. The spin dynamics is sluggish above and close to T g as in supercooled liquids and the system falls out of equilibrium below T g and shows aging as in a glass (1, 9). More recently, Tarzia and Moore (10) have paralleled the phenomenology of structural glasses to that of an Edwards-Anderson model in a uniform magnetic field. One of the main hurdles in making a direct real space connection between structural and spin glasses is that disordered spin models are defined on a lattice, whereas the particles comprising structural glasses are itinerant.Supercooled liquids and glasses are usually described within the Lagrangian formulation, in which one tracks the position of individual particles as a function of time. Natural quantities computed within this frame of reference are the particle's mean-square displacement and self-diffusion. Heterogeneous dynamics can be probed, for example, by studying quantities such as mobility within prescribed boxes; however, such fixed regions serve this purpose just for a certain time, because particles move in and out of these boxes if one waits for long enough. In contrast, studying local dynamics in a spin glass presents no such complication, because spins remain fixed to their sites at all times, and all that changes is the spin orientation as a function of time. Therefore, if one is to construct a simple d...