The Clean Air Act of 1990 was introduced to achieve and maintain a healthy environment while supporting strong and sustainable economic growth and sound energy policy. The federal regulations that followed require the Maritime shipping industry to work with government agencies to achieve these goals. This paper will estimate the theoretical stack emission from a vessel based on two models used by the maritime industry. The methods being discussed include one presented by the US Environmental Protection Agency (EPA) and the European Commission as done by Entec. Both models utilize published Lloyds Register Engineering Services Data to generate emission factors for each approach and therefore may yield good predictors of emissions. These models quantify both criteria pollutants and greenhouse gases. A comparison is made between the two methods with regard to similarity of emission rates from a model vessel. There are four parameters of NOx, SO2, CO2, and HC, which can be compared between the two methods. The values for NO2 in the EPA method would be equivalent to the NOx in the Entec methodology. The SO2 and CO2 values appear to be consistently higher in the EPA model. Hydrocarbon estimates appear to be consistently at least six times higher using the Entec model.