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Understanding the ionizing spectrum of low-metallicity galaxies is of greatimportance for modeling and interpreting emission line observations ofearly/distant galaxies.  Although a wide suite of stellar evolution, atmosphere, population synthesis,and photoionization models, taking many physical processes into account nowexist, all models face a common problem: the inability to explain the presenceof nebular HeII emission, which is observed in many low metallicity galaxies,both in UV and optical spectra. Several possible explanations have beenproposed in the literature, including Wolf-Rayet (WR) stars, binaries, verymassive stars, X-ray sources, or shocks. However, none has so far been able toexplain the major observations.  We briefly discuss the HeII problem, available empirical data, and observedtrends combining X-ray, optical and other studies. We present a simple andconsistent physical model showing that X-ray binaries could explain thelong-standing nebular HeII problem. Our model, described in Schaerer et al.(2019), successfully explains the observed trends and strength of nebular HeIIemission in large samples of low metallicity galaxies and in individualgalaxies, which have been studied in detail and with multi-wavelengthobservations. Our results have in particular important implications for theinterpretation of galaxy spectra in the early Universe, which will be obtainedwith upcoming and future facilities.

New insight on the far-UV SED and HeII emission from low metallicity galaxies