The Maslov index for Lagrangian pairs on ${\mathbb{R}}^{2n}$

We discuss a definition of the Maslov index for Lagrangian pairs on ${\mathbb{R}}^{2n}$ based on spectral flow, and develop many of its salient properties. We provide two applications to illustrate how our approach leads to a straightforward analysis of the relationship between the Maslov index and the Morse index for Sch"odinger operators on $[0,1]$ and $\mathbb{R}$.