This paper paper will examine some very different positions that Leibniz held or explored on monism, force, and substance during his long philosophical life. For reasons to be explained, positions drawn from Leibniz’s youth as well as his maturity will be considered. It will prove useful to consider these Leibnizian positions on these issues in relation to some of the leading alternatives of his age, in particular, those of Descartes, Spinoza and Malebranche. A guiding idea of this paper is that by meditating seriously on some key ideas explored by Leibniz on our topics, one can gain new insight into the philosophical options of his era. A few introductory remarks on Descartes, followed by an overview of the main divisions and aims of the present paper, will help set the stage for these Leibnizian meditations.