There is a substantial difference between the development methodologies of the following two Information Systems: Transaction Support System (TSS) and Decision Support System (DSS). The central concern of the Transaction Support System that supports the operational world in organizations is transaction processing. The retrieval of information for analytical purposes is in the very focus of the Decision Support System. The DSS provoked a series of changes in RDBMS technology. In 1995 the data warehousing was recognized as a new concept. Multi-dimensional architecture of a data model emerged beside the existing relational one. The following year brought an explosion of the OLAP tools. Now the time has come for the full recognition of the DSS development methodology as a separate discipline. This methodology is presented on the following pages. There are obstacles that make it difficult to accept or have a clear view of it. These obstacles are highlighted through the comparison to the TSS developmentmethodology that represents the traditional point of view. Tasks, deliverables, techniques, approaches, differences, similarities and analogies are discussed for each development activity. It is irrelevant for the discussion whether the DSS comprises data warehouse or/and some other store of data of the similar type, e.g. data mart or/and operational data store (ODS). This paper tries to reveal why it is so hard to accept the Decision Support System development methodology.