Starburst99 is a comprehensive set of model predictions for galaxies with active star formation, extending and improving upon the data set previously published by Leitherer & Heckman (1995). The models incorporate the latest stellar evolution models from the Geneva group and the model atmosphere grid by Lejeune et al. (1997). The models cover five metallicities between $Z = 0.040$ and 0.001, three initial mass function (IMF) choices, and age ranges from 10^6 to 10^9 years. The paper discusses the computational technique, model assumptions, and improvements over the previous version. It presents spectral energy distributions, ultraviolet line profiles, massive-star inventory, luminosities, colors, far-ultraviolet properties, and other diagnostic lines. The full data set is available electronically, and the source code is also provided. The models are useful for interpreting galaxy observations, particularly in comparing with spectral energy distributions.Starburst99 is a comprehensive set of model predictions for galaxies with active star formation, extending and improving upon the data set previously published by Leitherer & Heckman (1995). The models incorporate the latest stellar evolution models from the Geneva group and the model atmosphere grid by Lejeune et al. (1997). The models cover five metallicities between $Z = 0.040$ and 0.001, three initial mass function (IMF) choices, and age ranges from 10^6 to 10^9 years. The paper discusses the computational technique, model assumptions, and improvements over the previous version. It presents spectral energy distributions, ultraviolet line profiles, massive-star inventory, luminosities, colors, far-ultraviolet properties, and other diagnostic lines. The full data set is available electronically, and the source code is also provided. The models are useful for interpreting galaxy observations, particularly in comparing with spectral energy distributions.