Sommerfeld Theory Colloquium, In this lecture I propose to discuss some issues concerning the founda- tions of quantum mechanics and its interplay with space-time physics. I start by clarifying the distinction between ’realistic theories’ and ’probabilistic theories’ of Nature and sketch how the latter can often be viewed as ’deformations’ of the former. I then briefly recall some of the intriguing features of atomistic Quantum Mechanics, which belongs to the second class of theories. I attempt to describe, in con- ceptual terms, what it is that Quantum Mechanics predicts about Nature when appropriate experiments are done. I try to sketch some implications of this discussion for our views of space and time. I will conclude by sketching some recent results on the ’effective quantum dynamics’ of Open Systems, in particular on ’Quantum Brownian Motion’.