Water and life are inextricably connected. The energetic state of water (captured by thermodynamic state variables: chemical potential, activity, water potential and relative humidity) is the universally useful measure for the accessibility of water to participate in chemical and physical transformations. The broad relevance of water and the importance of its energetic state give rise to the interest in sensors which can measure the energetic state of water. In this dissertation, we present the microtensiometer (μTM) as a new tool for the direct measurement of water potential and as a microlaboratory for the study of the flow of aqueous solutions through porous media. After justifying the broad relevance of water, the energetic state of water and its measurement, we proceed to critically review methods for the measurement of the energetic state of water. In this review, we focus on the working principles, sample compatibility, accuracy, response time and range of these tools and methods. We argue that, due to the underlying physics and the relative ease of measuring pressure, methods which directly measure water potential are inherently more sensitive than other methods. We end this review with a discussion of challenges and opportunities in the measurement of the energetic state of water. We then present the μTM as a new tool for ex-situ and in-situ measurements of water potential. Here, our focus is on the development of packaging and measurement systems which promote full thermodynamic equilibrium between the μTM and the sample of interest. We demonstrate that the μTM can make measurements which agree with industry standard techniques. These results suggest the μTM is a first-of-its-kind in-situ probe of water potential. Before concluding, we present a study of osmotic flow with the μTM. The μTM allows us to examine osmotic transients in a rigid, nano-porous membrane permeable to water and solute. We present osmotic transients as well as a model which hypothesizes that these transients are influenced by an unprecedented time scale associated with the membrane reflecting solute. We also compare our results to an existing theory of osmosis in a permeable membrane.