**Essential Systems Analysis** was a new methodology published in 1984 by Stephen M. McMenamin and John F. Palmer for performing Structured Systems Analysis based on the concept of Event Partitioning.^{[1]}

The Essence of a system is "its required behavior independent of the technology used to implement the system".^{[2]} It is a model of what the system must do saying ideally nothing about how it will do it.^{[2]}

The methodology^{[1]} proposed that finding the true requirements for an information system entails the development of an Essential Model for the system, based on the concepts of a perfect internal technology, composed of:

- a perfect memory, that is infinitely fast and big, and
- a perfect processor, that is infinitely potent and fast.

It was later adapted by Edward Yourdon to develop Modern Structured Analysis.^{[3]}

The main result was a new and more systematic way to develop the Data Flow Diagrams, which are the most characteristic tool of Structured Analysis.

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^{a}^{b}McMenamin, Stephen M.; Palmer, John F. (1984).*Essential systems analysis*. Yourdon Press. ISBN 978-0-917072-30-7. - ^
^{a}^{b}Yourdon, Edward (2006).*Just enough structured analysis*. Ed Yourdon. **^**Yourdon, Edward. (1989).*Modern structured analysis*. Englewood Cliffs, N.J.: Yourdon Press. ISBN 0-13-598624-9. OCLC 17877629.