Stars account for the vast majority of the light we observe from the Milky Way and beyond. A single stellar population (SSP) is a group of stars formed at the same time and with the same initial composition. By combining superpositions of SSPs representing periods of star formation, we can model a wide range of galaxies. A long-standing issue when using empirical libraries to construct SSPs is non-arbitrary coverage of the HR-diagram, which imposes implicit age limits on SSP models that can be generated. The X-shooter Spectral Library (XSL) currently has poor coverage above Tef f ∼ 10000K, limiting its applicability to young SSPs. I will discuss current efforts to populate a previously bare region of the XSL HR diagram and thus extend the lower age limit for XSL stellar population models by up to a factor ∼10 for [Fe/H] = -1.2, and a factor ∼2 for solar metallicity. Massive stars are particularly interesting at low metallicities as possible progenitors of gravitational wave sources, long-duration gamma-ray bursts and sources of cosmic reionization in the early Universe. The Small Magellanic Cloud (SMC) at Z ∼ 0.2 Z provides the lowest metallicity environment where representative resolved populations of massive stars can be observed. I will discuss an ongoing project to find new high- significance candidate OB-type stars in the SMC. Using spectral energy distributions constructed from UV, optical and IR photometry, we have found hundreds of new candidate OB stars. Early results from spectro- scopic follow-up with Gemini-South are positive, thus potentially changing our view on the known massive stellar population in the SMC. Massive stars are also responsible for synthesising the majority of the short-lived radio-nuclides Al-26 and Fe-60 in the Galaxy. Upon decaying, they emit characteristic gamma rays which can be directly observed and mapped. While Al-26 can also be distributed by stellar winds, Fe-60 is only produced during a supernova. The comparison of both thus yields important constraints on the role and strength of stellar winds in evolved stars. I will talk about the importance of fully understanding the underlying nuclear reaction rates that create and destroy Al-26 in massive stars in order to gain a better perspective of the role of stellar winds and their yields.