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Periodic cellular materials can have superior mechanical effective properties such as higher stiffness, strength, and/or flexibility compared to other materials with similar density. There is a significant body of research focusing on designing meso scale periodic cellular material structures with superior effective elastic properties; however, there is only limited research on how to guide designers on developing improved unit cell topologies and shapes, for a given set of loading requirements and conditions. To address this challenge, this dissertation introduces unit cell design guidelines. The guidelines codify design knowledge to provide direction and recommendations to engineers who are trying to change the topology and shape of unit cells to improve a targeted measure of performance. The guidelines investigate the effects of changes to topology and shape characteristics of unit cells subjected to in-plane shear loading. The investigated topology and shape characteristics include side connections, transverse connections, curved beams, and vertical legs. In order to validate that the unit cell design guidelines apply to a variety of unit cells with similar topology characteristics, each guideline is applied to a variety of unit cells and numerical simulations are used to validate that the performance of the unit cell is improved by the change. More importantly, this dissertation introduces a formalized and systematic method to develop unit cell design guidelines to achieve desired mechanical effective properties of a meso scale periodic cellular materials by changing the topology and shape of a unit cell. In order to support unambiguous discussion of cellular topology, a vocabulary is developed to describe two-dimensional periodic cellular materials. The vocabulary represents the 2D periodic cellular materials through descriptions of the unit cell and of the tiling of the unit cell arrangements. A designer study is conducted to evaluate the usability of the vocabulary by engineers. The results of designer study support the idea that the vocabulary is usable by engineers who generally do not have a background in periodic cellular materials. Two user studies are conducted to evaluate the effect of subjectivity and sequencing of the guidelines. The subjectivity results show that the modified unit cells developed by different engineers using the same guideline and initial topology have similarities. However, the degree of similarity is affected by the content of the guidelines, how the guidelines are written, and the initial unit cells. The sequencing of guidelines is also investigated to evaluate if changing the order of guidelines affects modified unit cells. Ultimately, the guidelines are validated through numerical simulations and controlled studies with human subjects while the guideline development method is demonstrated through one case study with novice engineer developing new guidelines under tension loading.