Micromechanics is a set of techniques for calculating the average response of heterogeneous materials from the constituents properties, shape and arrangement. It plays a crucial role in the analysis, design and optimization of new materials, revealing how local properties and geometry influence the overall response. While the history of micromechanics is rich with models proposed during the past 50 years, the search continues for an ideal model. This book describes the original construction of a recently-proposed micromechanics approach called the finite-volume direct averaging micromechanics theory, which is an attractive alternative to the widely used finite-element based homogenization of periodic materials. It includes a brief overview of different micromechanics models, detailed construction of the finite-volume theory s framework, and applications involving elastic-plastic response of unidirectionally-reinforced materials. The presentation is accessible across interdisciplinary boundaries to engineers and research scientists involved in the development of advanced material systems, or interested in learning how local constituent properties percolate to the macrolevel.