Abstract: I introduce here our recent work on studying the transition between atomic and molecular ISM. We developed an observing technique, HI Narrow Self-Absorption (HINSA), which provides a rare "robust chemistry clock”. Using HINSA, we measured the molecular cloud formation time scale to be ~10 Myr, thus more consistent with the canonical star formation picture than with fast star formation scenarios. We also observed and modeled C+, OH, and CH in transition zone and clouds and derived empirical formulae for OH abundance, dark gas density, etc. versus extinction. Simple hydrides have much tighter X-factor than CO, making e.g. CH and OH, better tracer of H2 in intermediate extinction regions. We also identified a clear example of an isolated dark clouds undergoing current H2 formation. Our studies have implications in understanding star formation law, simulating galaxy evolution, and protostellar disk formation.