Negative mass is a peculiar concept: push an object with negative inertial mass and it will accelerate ***towards*** the pushing force. This counterintuitive behaviour can be realized in Bose Einstein condensates by using a lasers to manipulate the underlying dispersion relationship through a static gauge field referred to as a spin-orbit coupling (SOC). This leads to an array of interesting phenomenon, including the breaking of Galilean covariance, modulational instabilities, and self-trapping. In this talk I will discuss an experiment performed at WSU wherein Rb87 atoms are cooled and manipulated to demonstrate negative effective mass. The experiment is well described by a set of negative-mass hydrodynamic equations, which are then used to demonstrate that features like self-trapping observed in optical lattice experiments can be simply described in terms of negative effective mass without complications from the spatial dependence of the lattice potential.