Most stars in our Galaxy form in clusters which often contain the massive stars which, with their prodigious energy output, dominate the evolution of the interstellar medium. Understanding how these clusters and the massive stars within them form and evolve is one key questions for astrophysics. A major challenge is identifying the clumps of dense gas in molecular clouds which give rise to these clusters before star formation significantly modifies their properties. Infrared dark clouds (IRDCs), dense regions seen in absorption against the diffuse infrared emission in the Galactic Plane, are amongst the best candidates for identifying such objects. In this talk I will draw on IRDCs identified in a recent new catalogue, plus other recent data, to explore how the gas in molecular clouds gathers to form massive dense clumps and how these subsequently evolve as star formation takes place within them.