With high refractive index media and high NA lenses, the Abbe limit is roughly 200 nm (0.2 [micro]m) in the X-Y plane.
Before discussing newer optical methods that achieve molecular-level resolution, it is worth looking at some useful techniques that remain bound by the Abbe limit.
The Abbe limit refers to resolution in the X-Y plane.
If there can be a distinction between the 2 objects in the spectra of their emitting photons, or their temporal release of photons, the Abbe limit does not apply, and in fact there is no theoretic limit to light microscopy resolution, even in living cells!