Ultra-compact HII regions

This article reviews how observations of dense molecular gas can help us understand the dynamics and evolution of ultra-compact H II (UCHII) regions. Particular attention is paid to the cometary objects, which it is argued are likely to be the dominant class. Thermal molecular lines in emission and absorption are considered as well as the mainly dynamical information from masers. Most of the UCHII regions studied at high resolution have hot cores ahead them which makes interpretation of molecular observations much more complex. Significant support for the champagne flow model for cometary regions can been seen, adding to the evidence from studies of the ionised gas. How the prevalence of cometary objects may fit into a triggered, sequential picture of massive star formation is outlined.