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In the early 1930’s, Taylor discovered that viscous flow between two closely-spaced rotating concentric cylinders becomes unstable whenever the dimensionless number (where Re is the Reynolds number, d is the gap spacing, and R1 is the radius of the inner cylinder) exceeds 41. Similarly, in the late 1930’s, Dean discovered that pressure-driven flow between two fixed concentric cylinders also becomes unstable whenever the same dimensionless number exceeds 36. Both types of instability were realized to be of significant technological importance. For this reason, they have both been the subject of so much study in the past [see the paper reviews in Refs. 3 and 4]. However, while the main application of Taylor instability appears to be in relation to Couette viscometers, application of Dean instability appears to be much broader encompassing heat and mass transfer enhancement, chaotic mixing, microfluidics, and microfiltration, among others. The flow is also of fundamental importance in studies related to hydrodynamic instability theories. Thus, it is of no surprise that Dean instability has been investigated so extensively in the past, for both Newtonian and non-Newtonian fluids alike. As to the Dean instability of non-Newtonian fluids, most studies carried out in the past are in relation to viscoelastic fluids or purely-viscous fluids. In contrast, Dean instability of viscoplastic fluids has not been investigated to the same extent. This is quite surprising realizing the fact that viscoplastic (or yield-stress) fluids are quite common in industry. One can notably mention foodstuffs, pharmaceutics, cosmetics, paints, and drilling muds, among others. A well-known model to represent yield-stress fluids is the Bingham model. Instability of Bingham fluids has been addressed in Taylor-Couette flow , in plane Poiseuille flow, and in annular pressure-driven flow. Similarly, Dean instability of Bingham fluids has been investigated in curved pipes by Das, and in curved rectangular ducts by Fellouah. The latter work relied on the FLUENT software to show that the onset of Dean instability is delayed by an increase in the Bingham number. To the best of our knowledge, instability of Bingham fluids in azimuthal pressure-driven flow between two infinitely-long concentric cylinders has not previously been addressed using normal mode analysis. In the present work, we are going to investigate hydrodynamic instability of Bingham fluids in pressure-driven flow between two fixed concentric cylinders of infinite length. Our approach is a classical one in which we superimpose an infinitesimal perturbation to the basic flow and see how its amplitude grows in time (the so-called temporal instability analysis). An eigenvalue problem will be Dean Instability of Bingham Fluids in Tangential Flow between Two Fixed Concentric Cylinders