Galileo Galilei invented the first microscope “occhiolino,” by combining a concave and a convex lens in 1600s. Robert Hooke and Anton van Leeuwenhoek later modified it to look at living things. Since then, light microscopy has gained immense popularity and has been pushing the limits of optical technology. The race to improve the power of seeing continued by introduction of new techniques, more and more powerful lenses, better optical corrections, stronger light sources, higher-sensitivity detectors, and assembly of cutting-edge systems. However, the wish for a better peek at the cellular world hit a wall in the 1990s, already envisioned by Ernst Abbe in 1873. Diffraction, which enables an image to be formed in the first place, is also a barrier that obscures the finer details of cells and biomolecules. This chapter presents a discussion of resolution, briefly introducing some of the fundamental concepts such as diffraction and contrast.