This chapter discusses a matrix exponential spatial specification (MESS) introduced by LeSage and Pace (2007). As discussed in Section 7.6, one can view MESS as arising from a spatiotemporal process with exponential decay of influence from previous periods. The matrix exponential can provide an alternative to the spatial autoregressive process as a basis for building spatial regression models. It essentially replaces the geometric decay over space associated with the spatial autoregressive process with exponential decay over space.