Individual human cancer cells often show different responses to the same treatment. In this talk I will share the quantitative experimental and computational approaches my lab has developed for studying the fate and behavior of human cells at the single-cell level. I will focus on the tumor suppressor protein p53, a transcription factor controlling genomic integrity and the response to DNA damage. In the last several years my lab has established the dynamics of p53 (changes in its levels over time) as an important mechanism controlling gene expression and cellular outcomes. In response to double-strand DNA breaks, p53 exhibits pulses of expression that allow cells to repair the damage and resume growth. Switching these pulses into a sustained response enhances the activation of terminal fates, such as apoptosis and sentences. I will present studies from the lab demonstrating how studying p53 dynamics in response to radiation and chemotherapy in single cells can guide the design and schedule of combinatorial therapy. I will also present new findings using a combination of digestion-free mass spectrometry on intact p53 proteins and global transcriptional profiling, suggesting that p53’s post-translational modification state is altered between its first and second pulses of expression, and the effects these have on gene expression programs. Such an interplay between dynamics and modification may offer a strategy for hubs proteins like p53 to temporally coordinate multiple cellular processes in cells.