Starburst99 is a comprehensive set of model predictions for spectrophotometric and related properties of galaxies with active star formation. The models are an improved and extended version of the data set previously published by Leitherer & Heckman (1995). The models are based on the latest stellar evolution models from the Geneva group and the model atmosphere grid from Lejeune et al. (1997). The models are presented for five metallicities (Z = 0.040 to 0.001) and three initial mass functions (IMFs). The age coverage is from 10^6 to 10^9 years. The full data set is available for retrieval at http://www.stsci.edu/science/starburst99/. The website allows users to run specific models with non-standard parameters and access the source code. The paper discusses synthesis models for galaxies with active star formation, focusing on their properties and the computational techniques used. The models consider two star-formation laws: an instantaneous burst and continuous star formation. Three IMF parameterizations are used, including a Salpeter IMF, a steeper IMF, and a truncated IMF. The models are optimized for massive stars and are used to interpret galaxy observations. The models include spectral energy distributions, ultraviolet line profiles, stellar inventories, luminosities, colors, far-ultraviolet properties, and other diagnostic lines. The models are used to study the properties of star-forming regions and galaxies, including the effects of metallicity, stellar evolution, and the contribution of different stellar populations. The models also consider the impact of nebular continuum and the effects of stellar winds and supernovae on the interstellar medium. The paper concludes that the models provide a valuable tool for understanding the properties of galaxies with active star formation. The models are available online and can be used by the community for further research and analysis. The models are based on the latest stellar evolution and atmospheric models, and they are designed to be useful for a wide range of applications in galaxy studies.Starburst99 is a comprehensive set of model predictions for spectrophotometric and related properties of galaxies with active star formation. The models are an improved and extended version of the data set previously published by Leitherer & Heckman (1995). The models are based on the latest stellar evolution models from the Geneva group and the model atmosphere grid from Lejeune et al. (1997). The models are presented for five metallicities (Z = 0.040 to 0.001) and three initial mass functions (IMFs). The age coverage is from 10^6 to 10^9 years. The full data set is available for retrieval at http://www.stsci.edu/science/starburst99/. The website allows users to run specific models with non-standard parameters and access the source code. The paper discusses synthesis models for galaxies with active star formation, focusing on their properties and the computational techniques used. The models consider two star-formation laws: an instantaneous burst and continuous star formation. Three IMF parameterizations are used, including a Salpeter IMF, a steeper IMF, and a truncated IMF. The models are optimized for massive stars and are used to interpret galaxy observations. The models include spectral energy distributions, ultraviolet line profiles, stellar inventories, luminosities, colors, far-ultraviolet properties, and other diagnostic lines. The models are used to study the properties of star-forming regions and galaxies, including the effects of metallicity, stellar evolution, and the contribution of different stellar populations. The models also consider the impact of nebular continuum and the effects of stellar winds and supernovae on the interstellar medium. The paper concludes that the models provide a valuable tool for understanding the properties of galaxies with active star formation. The models are available online and can be used by the community for further research and analysis. The models are based on the latest stellar evolution and atmospheric models, and they are designed to be useful for a wide range of applications in galaxy studies.