We collect a number of derivations and interpretations of Gelfand-Levitan-Marrenko (GLM) equations in various contexts. Relations to Riemann-Hilbert (RH) problems, tau functions and determinants, direct linearization techniques, etc. are discussed and some new results are included. We emphasize the role of spectral pairings of generalized eigenfunctions and exhibit various ways in which inverse scattering techniques arise. The natural extension and generalization of GLM techniques in the format of RH methods and coadjoint orbits in a Lie theoretic context is developed and the hierarchy point of view is explored in terms of spectral ideas. Thus, the roles and some limitations of the GLM and RH methods are exposed to some extent and some bridges are sketched to modern techniques in soliton mathematics involving hierarchies, tau functions, Lie theory, etc.