Early efforts in formation evaluation led to the conclusion that well log responses available at the time did not provide unique solutions for determining whether or not an interval contained producible hydrocarbons. Also, deriving accurate estimates of permeability was problematic. Workers recognized that when additional data of the rock properties were available, reasonable estimates of producibility and permeability could be reached. A scheme of rock typing was developed which is based on the premise that rocks of similar physical characteristics, such as textures and cementation, have similar petrophysical properties. The application of rock typing and rock catalogs to formation evaluation remains a powerful tool to estimate whether a unit will flow. This presentation provides an introduction to rock typing and demonstrates its application via two case studies from the Deepwater Gulf of Mexico. The case studies show where time-critical rock-typing contributed to decisions to i) continue drilling in one case and ii) to complete a well in the other case.