When a dispersion containing spherical colloids is mixed with a polymer solution two kinds of instabilities can occur: bridging flocculation (1) caused by adsorbing polymer or unmixing driven by the depletion force (2). The type of instability encountered depends on whether the polymers adsorb onto the colloidal surfaces. Polymer adsorption occurs when the contact between the colloid surface and the polymer segments is energetically favorable to such a degree that the loss of configurational entropy is compensated. When the amount of adsorbing polymer in the system does not suffice to fully cover all available surface area on the colloids, so-called bridging flocculation occurs. Some polymers then attach to more than a single particle leading to aggregates or complexes, which tend to sediment when they are large (situation '1'). Such a flocculation with both colloids and polymers concentrated in one part of a container indicates polymer adsorption. When all particle surfaces are saturated with adsorbed polymers in a good solvent, the particle interactions are effectively repulsive because dense polymer layers overlap upon close approach giving rise to steric repulsion, which kinetically stabilizes the dispersion.