Current concerns about climate change, energy security and record high oil prices have triggered high enthusiasm and push for plug-in vehicles. Widespread adoption of plug-in vehicles would result in significant reductions in CO2 emissions from transportation. It would also reduce our dependence on fossil fuels by replacing petroleum-sourced energy with renewable, domestically produced electricity.While a few OEMs have successfully launched hybrid vehicles and even toyed with plug-in hybrid solutions in the passenger car market segment, little attention has been placed on heavier commercial vehicles.Large utilities operate fleets of several hundred diesel-power trouble trucks to repair and maintain their transmission and distribution infrastructure. Medium-duty segment is over a million vehicles annually. These vehicles are typically driven in densely populated neighborhoods. Idling emissions and noise from Utility service vehicles are disruptive to residential areas and expose operators to diesel exhaust for extended periods.Plug-in Hybrid technology offers a number of advantages to the traditional utility truck. Using stored battery energy, the vehicles can operate extended hours continuous, running the bucket, power tools, lights and accessories while the engine is turned off.In 2006, the Electric Power Research Institute (EPRI) and Eaton Corporation partnered in an initiative to develop the first medium duty Diesel plug-in hybrid vehicle, retrofitting a Ford class V F550 trouble truck with Eaton's hybrid power train with a grid-rechargeable 16kWh lithium ion battery.This paper presents the design, development and testing results of this Plug-in Hybrid Vehicle and details the mechanical and electrical integration of the hybrid drive train, high energy batteries and on-board charging system, to provide quiet and emissions-free standby operations and hybrid operation while driving.This paper also discuss the benefits and trade-offs of the system architecture, battery sustaining / depleting strategy and estimated impact on the grid.