Production of electricity by friction is well known but poorly understood, and is the source of electrostatic discharge causing serious accidents. Recent results are in agreement with one of the conflicting views on this problem, according to which triboelectricity in polymers is triggered by mechanochemical and wear or mass transfer phenomena. These results also challenge the widely accepted paradigm of one-way charge transfer that is the basis of the triboelectric series. Experimental results from powerful analytical techniques coupled to surface charge mapping support the following hypothesis: charge-bearing species are ionic polymer fragments formed through mechanical action. Beyond this, the atmosphere participates through tribocharge build-up and dissipation due to reactive plasma formation and charge exchange at the gas-solid interface, mediated by adsorption of non-neutral water, or ion partition during water adsorption, as in hygroelectricity phenomena. Mechanochemical reactions during polymer friction or contact produce ionic fragments distributed on positive and negative domains at both surfaces.