Single cells sense their environment with remarkable precision. At the same time, cells have evolved diverse mechanisms for communicating. How are sensing and communication related? I will describe recent theoretical and experimental results in which this question is explored in several contexts, including gradient detection by connected epithelial cells, and collective invasion of breast cancer cells. I will show how communication allows cells to perform qualitatively new behaviors that single cells cannot perform alone. Moreover, I will demonstrate that minimal mathematical modeling yields fundamental limits to the precision of sensing, and that these limits are critically altered by cell-to-cell communication. This work extends the study of cellular sensing and information processing to collective ensembles.