This paper summarizes the results of an extensive experimental study aiming to evaluate the performance and pollutant emissions of diesel and CNG waste collection trucks under realistic and controlled operating conditions in order to support a fleet renewal initiative in the city of Milan. Four vehicles (1 diesel and 3 CNG) were tested in two phases using a portable emission measurement system. The first phase included real world operation in the city of Milan while the second involved controlled conditions in a closed track. Emissions recorded from the diesel truck were on average 2.4kg/km for CO₂, 0.21g/km for HC, 7.4g/km for CO, 32.3g/km for NO_x and 46.4mg/km for PM. For the CNG the values were 3.6kg/km for CO₂, 2.19g/km for HC, 15.8g/km for CO, 4.38g/km for NO_x and 11.4mg/km for PM. CNG vehicles presented an important advantage with regards to NO_x and PM emissions but lack the efficiency of their diesel counterparts when it comes to CO, HC and particularly greenhouse gas emissions. This tradeoff needs to be carefully analyzed prior to deciding if a fleet should be shifted towards either technology. In addition it was shown that existing emission factors, used in Europe for environmental assessment studies, reflect well the operation for CNG but were not so accurate when it came to the diesel engine truck particularly for CO₂ and NO_x. With regard to NO_x, it was also shown that the limits imposed by current emission standards are not necessarily reflected in real world operation, under which the diesel vehicle presented almost 4 times higher emissions. Regarding CO₂, appropriate use of PEMS data and vehicle information allows for accurate emission monitoring through computer simulation.