Colloid-colloid interactions are important in understanding the macroscopic properties of flowing suspensions. In many processes of technological and biological interest, it is important to identify conditions which promote or inhibit colloidal aggregation. In this paper, we use trajectory amalysis to understand the effect of hydrodynamic and nonlydrodynamic forces on colJoidal stability. All linear hydrodynamic fows can be represented in a tinite region of the or (\mathrm{L}^{2})-det (\mathrm{I}), plane, where (\mathrm{L}), is the normalized velocity gradient tensor with constant magnitude. We calculate the stability ratio (W) for different flow types, specified by (\mathrm{tr}^{2}) and det (\mathrm{L}). For purely attractive interparticle potentials, a small region around simple shear flow (tr (\mathbf{L}^{2}=0, \operatorname{det} \mathbf{L}=0) ) shows uniquely high stability. Small changes in (\operatorname{tr} \mathbf{L}^{2}) or det (\mathbf{L}), which are equivalent to changes in the relative magnitude of vorticity or the relative orientation between vorticity and extension, cause a great decrease in stability. Away from simple shear flow, (W) is independent of changes in flow type for the entire class of linear flows with open streamlines ( (\operatorname{tr} \mathrm{L}^{2} \geqslant 0) ). Interparticle potentials with primary and secondary minima exhibit the same stability as purely attractive potentials as long as the Debye screening length is less than a critical valuc. Greater Debye lengths lead to complete stability ((W \rightarrow \infty)). The critical Debye length depends on fluid flow type and the value of inverse critical Debye length correlates with the strength of the flow field. The magnitude of the particle surface potential has little effect on the stability ratio. Taken together, the results show the type of hydrodynamic flow to be an important determinant of the aggregation behavior of colloidal particles. Furthermore, aggregation in simple shear flow is different than that in other linear flows, and we caution against extrapolating aggregation behavior in simple shear to more complex fluid llow situations. as 194 Academic Press. Inc.