Cellular patterning in development is the process by which cells differentiate from one another into distinct cell types. Patterning is essential for multicellular organisms to split tasks between different cells to achieve complex functions. The ancestors of animals and plants developed multicellularity independently, so it is important to study cellular patterning in both kingdoms of life. Here, I focus on cell patterning mechanisms in the plant Arabidopsis thaliana. Specifically, I seek to understand how epidermal pavement cells of different sizes are patterned during development. Sepal giant cells are very large, highly endoreduplicated cells interspersed among much smaller cells on the sepal epidermis. Between sepals, giant cell localization differs but giant cell number is fairly consistent. Several genes involved in embryonic epidermal specification have been found to also control giant cell formation, including ATML1 which encodes a class IV homeodomain leucine zipper transcription factor. ATML1 protein levels fluctuate within epidermal nuclei of developing sepals and high concentrations reached in the G2 phase of the cell cycle is strongly correlated with giant cell fate specification. I found that the receptor-like kinase ALE2 functions upstream of the transcription factor ATML1 by affecting its activity and preventing it from properly increasing transcription of the CDK inhibitor LGO at the right time and to the right level for giant cell specification. Thus, I find that cell-cell signaling is necessary to sensitize cells of the developing sepal epidermis to endogenous ATML1 concentration peaks. I found that levels of PDF2, the paralogue of ATML1, also fluctuate in individual nuclei of the developing sepal epidermis and that its fluctuations are sometimes correlated with ATML1 fluctuations but at times are not correlated. Like ATML1, increased PDF2 concentration promotes giant cell differentiation. The fact that ATML1 and PDF2 fluctuations are sometimes very correlated suggests either that they regulate the expression of one another or that they are responding to the same variations in cellular environment. The Arabidopsis thaliana leaf epidermis also has a range of cell sizes and ploidies. I found that the same genetic pathway that controls cell size in the sepal also controls cell size in the leaf. With collaborators Gauthier Weissbart and Pau Formosa-Jordan at the Max Planck Institute for Plant Breeding Research, I revealed an underlying order to the spatial arrangement of leaf giant cells that initially appear randomly distributed. I found that giant leaf cells are scattered nonrandomly and touch one another more than would be expected by chance. I show that this spatial pattern can be produced by a model in which giant cells are specified by stochastic fluctuations of ATML1.