Keywords: Colloidal dispersions, Brownian motion, rheology Abstract: The motion of a single individual particle in a complex material is fundamental to understanding the dynamical properties of the material. Monitoring such motion has given rise to a suite of experimental techniques collectively known as ‘microrheology,’ with the ability to probe the viscoelastic properties of soft heterogeneous materials (e.g. polymer solutions, colloidal dispersions, biomaterials, etc.) at the micrometer (and smaller) scale. In microrheology, elastic and viscous moduli are obtained from measurements of the fluctuating thermal motion of embedded colloidal probes. In such experiments, the probe motion is passive and reflects the near-equilibrium (linear response) properties of the surrounding medium. By actively pulling the probe through the material one can gain information about the nonlinear response, analogous to large-amplitude measurements in macrorheology. But what exactly is measured in a microrheological experiment? And how does the micro-rheological response compare with conventional macrorheology? To answer these questions, we consider a simple model – a colloidal probe pulled through a suspension of neutrally buoyant bath colloids – for which both micro- and macro-results can be obtained exactly. The moving probe distorts the dispersion’s microstructure resulting in a reactive entropic or osmotic force that resists the probe’s motion, which can be calculated analytically and via Brownian Dynamics simulations and used to infer the dispersion's 'effective microviscosity.' By studying the fluctuations in the probe’s motion we can also determine the force-induced 'micro-diffusivity.' Connections between micro and macro behavior will be explored.