High-speed video microscopy has been used to study drop breakup in dilute Newtonian emulsions under steady shear. Fundamental experimental studies on drop breakup have been limited to breakup in quiescent matrix or under pseudo-equilibrium conditions. This thesis represents the first direct visualization of drop breakup under steady shear at high capillary numbers (Ca). The mechanisms of drop breakup depend on Ca and the viscosity ratio (lambda). At Ca ∼ Cac , drops are broken up via necking. At Ca < 2Cac, drop breakup is caused by end pinching. At Ca > 2Cac, the capillary instability is the dominant breakup mechanism. For Ca > 2Cac, breakup dynamics are strongly controlled by lambda. For 0.1 < lambda < 1, drops with different initial sizes deform into threads with the same radius at breakup. The wavelength of the capillary instability is uniform along the length of a thread and from thread to thread. Fairly monodisperse dilute emulsions are obtained due to this size selection mechanism, with the average drop size being inversely proportional to the shear rate. For 1 < lambda < 3.5, the breakup mechanism is similar to that for 0.1 < lambda < 1.0, except that the satellite drops are substantially larger, resulting in polydisperse emulsions. For lambda < 0.1, the daughter drops are formed from long wavelength capillary instability and may break again. This induces collisions between drops, which in turn results in irregular drop re-breaking and coalescence, producing polydisperse emulsions. This re-breaking mechanism has not been observed in previous studies in the literature. Drops reach a pseudo-steady state before the capillary instability starts to grow. At this pseudo-steady state, the shear stress and the capillary pressure almost balance each other, determining a definite thread radius, which is independent of the initial drop size. We define a dimensionless thread number as the ratio of the two forces. The thread number is only a function of lambda, and shows a minimum in lambda. The measured thread number is in agreement with the slender body theory of Hinch and Acrivos (1980). Drops deform pseudo-affinely for 0.1 < lambda < 1.0, but deformation deviates from being pseudo-affine otherwise.