The development of systems ranging from embryos to metastases is governed by regulatory factors that control specification trajectories and by cell division patterns that determine the lineage relationships of cells. I will illustrate how single-cell RNA sequencing and CRISPR-Cas9 genome editing provide new tools to reconstruct developmental trajectories and lineage trees at very large scales. I will discuss the opportunities and challenges for these technologies to provide new views of development (for more information see McKenna et al. Science 2016; Farrell et al. Science 2018; Raj et al. Nature Biotechnology 2018).