I will describe the application of an energy transfer technique (NanoBRET) that enables an approach to broadly profile compound fractional occupancy and residence time against a variety of target classes inside intact, living cells. Using this method, broad spectrum evaluation of compound engagement can be measured against over 300 full-length human kinases in live cells. Target engagement potencies correlate strongly with potencies using more traditional pathway analysis readouts, thus providing a platform to establish structure-activity relationships (SARs) for kinase chemical probes or lead drug molecules. In live cells, we have uncovered a surprising spectrum of intracellular activity for certain cyclin dependent kinase inhibitors (CDKi’s) and PROTACs, offering opportunities for repurposing some chemotypes as selective chemical probes for understudied kinases. We further evaluate opportunities for achieving target selectivity under non-equilibrium cell culture conditions, via protracted target residence time or PROTAC-mediated degradation. Here, we describe a broadly applicable approach for evaluating existing and novel chemical matter for selectively engaging CDKs in living cells.