This EPA staff technical paper presents a preliminary staff assessment of the feasibility of upcoming emissions standards for nonroad diesel engines. These standards, referred to as 'Tier 3' for engines above 50 horsepower (hp) and 'Tier 2' for engines below 50 hp, were adopted in a 1998 rulemaking, but do not begin to take effect until the middle of this decade. EPA announced in the 1998 rule that we planned to perform this assessment to determine the need for any adjustments to the program. We are soliciting public comments on this preliminary assessment. After reviewing the comments, EPA plans to issue a final assessment early next year on these issues. This paper also discusses continuing air quality concerns caused by nonroad diesel emissions after implementation of the upcoming Tier 3 program. We intend to follow up with a rulemaking to address these concerns next year. Furthermore, that rulemaking will fulfill Agency commitments, made in the 1998 final rule, to establish test procedures and standards levels for controlling PM emissions beyond the limited control achieved under the Tier 2 standards, and to adopt measures to better ensure emissions control in-use.