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Star formation in the early universe was characterized by the prevalence of massive stars, shaped by the thermal and chemical properties of low-metallicity gas. I will begin by discussing the formation of the first stars based on radiation hydrodynamics simulations, which show that they typically grow to tens to hundreds of solar masses, often forming in binary or multiple systems. Next, I will examine the gradual evolution of the initial mass function (IMF) with increasing metallicity. Dust-induced cooling becomes effective around 10^-5 to 10^-4 Zsun, allowing low-mass star formation, though massive stars still dominate. As metallicity increases, the IMF shifts toward a bottom-heavy distribution, approaching a Salpeter-like form near 0.1 Zsun. I will also discuss the role of CMB heating at redshifts z greater than 10, which suppresses gas cooling and leads to a more top-heavy IMF, increasing the number of supernovae by a factor of a few compared to the Chabrier IMF at Z around 0.1 Zsun. Finally, I will consider the implications for the unexpectedly large number of UV-bright galaxies observed by JWST at z greater than ~10. Our simulations suggest that star cluster formation at metallicities around 10^-2 to 10^-3 Zsun, where the IMF remains top-heavy and radiative feedback is not yet too strong, can account for the high UV luminosities, as the star formation efficiency remains sufficiently high.