The use of microcantilevers in theological measurements of gases and liquids is demonstrated. Densities and viscosities of both gases and liquids, which can range over several orders of magnitude, are measured simultaneously using a single microcantilever. The microcantilever technique probes only minute volumes of fluid (< 1 nL), and enables in situ and rapid rheological measurements. This is in direct contrast to established methods, such as "cone and plate" and Couette rheometry, which are restricted to measurements of liquid viscosity, require large sample volumes, and are incapable of in situ measurements. The proposed technique also overcomes the restrictions of previous measurements that use microcantilevers, which are limited to liquid viscosity only, and require independent measurement of the liquid density. The technique presented here only requires knowledge of the cantilever geometry, its resonant frequency in vacuum, and its linear mass density. A simple yet robust calibration procedure is described to determine the latter two parameters, from a single measurement of the resonant frequency and quality factor of the cantilever in a reference fluid (such as air), if these parameters are unknown.