Understanding the molecular phase of the ISM in starburst and active galaxies is important for the modelling of the onset and evolution of their nuclear activity. Observations of high density gas tracers such as HCN, HNC, HC3N, HCO+ and CN are essential for probing physical and chemical conditions of the dense, star-forming gas. These tracers show great potential as indicators of the evolution of star formation as well as probes of X-ray illuminated molecular gas around an active galactic nucleus (AGN). In particular, towards the inner kpc of luminous and ultra luminous galaxies will molecular line ratios prove useful as diagnostic tools, since optical and even near infrared starburst tracers are difficult to apply in these highly obscured regions.