Over the last decade, particle tracking microrheology has matured as a new tool for complex fluids research. The main advantages of microrheology over traditional macroscopic rheometry are: the required sample size is extremely small (~ 1 mL); local viscoelastic properties in a sample can be probed with high spatial resolution (~1-10 mm); and the sample is not disturbed by moving rheometer parts. I will present two examples of recent work in my group that highlight how these characteristics can be exploited to acquire unique information about the microstructure of complex fluids. First, we have studied protein unfolding. Traditionally, protein unfolding is studied with spectroscopic techniques (circular dichroism, NMR, fluorescence). Although viscosity has been listed in textbooks as a suitable technique, few-if any-quantitative rheological studies of unfolding have been reported, mainly due to technical difficulties. With microrheology, we have been able to quantify the size of the folded and unfolded protein, as well as the Gibbs free energy of unfolding, for aqueous bovine serum albumine solutions upon addition of urea as a denaturant. The results are in excellent agreement with literature data.