Massive stars are known to be formed in clusters. To understand their birth process it is essential to know the physical conditions of the parental cloud which is thought to play a critical role in determining the formation mechanism. In this contribution I summarize recent results from observations of dust, molecular and ionized gas emission we have made toward several massive star forming regions in the southern hemisphere. These observations are providing key evidence concerning the initial conditions for the formation of cluster of massive stars, allowing to characterize the physical properties of massive and dense cores and permitting to identify them in different stages of their early evolution.