Method of Fluxions (Latin: De Methodis Serierum et Fluxionum)[1] is a mathematical treatise by Sir Isaac Newton which served as the earliest written formulation of modern calculus.

He originally developed the method at Woolsthorpe Manor during the closing of Cambridge due to the Great Plague of London from 1665 to 1667.

For a period of time encompassing Newton's working life, the discipline of analysis was a subject of controversy in the mathematical community.

Instead, analysts were often forced to invoke infinitesimal, or "infinitely small", quantities to justify their algebraic manipulations.

Some of Newton's mathematical contemporaries, such as Isaac Barrow, were highly skeptical of such techniques, which had no clear geometric interpretation.