Public transport has been considered the preferred strategy to reduce congestion and pollution from urban road traffic. For low to medium capacity, bus systems are considered the most affordable and flexible mode. Currently, diesel based systems still dominate transit bus market, due to their high productivity, low deployment costs, technological maturity, operational reliability and flexibility (high daily ranges, fast refuelling and no infrastructure requirement along the routes). However, although some important improvements in engine technology and aftertreatment devices, enforced by emission standards improvements (Euro VI, US 2010 and those related), have been achieved, it is well known that there is a limit to cleaning exhaust diesel buses exhaust. In this context, transit authorities and operators have been under pressure to shift for more environmental friendly technologies. Electric traction meet deserved operational and environmental features, with its high motor efficiency (allied with regenerative braking), torque (specially desired for stop and go cycles and hilly cities), low/zero emission (dependent on electricity mix) and low noise levels. Trolleybus systems are considered a well known and proven electric driven technology, that have been intensively used in the past, but that have been discontinued due to the massification of internal combustion engine vehicle use, with its inherent operational flexibility, albeit with an environmental burden, that was not appropriately weighted until recently. Currently, with an increasingly pressure to the adoption of environmental friendly transport systems, as well as due to technological/operational improvements (hybrid trolleybus configurations - which allows a free overhead wire range, ac motors, regenerative brake approach) trolleybus reemerges as an environmental friendly (zero local emission) transport system for medium to high loaded corridors, as Bus Rapid Transit - BRT. This is specially true for countries with a large share of renewable energy into electricity generation mix. This work presents a technical overview of modern trolleybus systems, with a detailed assessment of technological features of vehicles and infrastrucuture, as well as an economical evaluation focused on their fixed and variable costs, followed by a SWOT matrix analysis. Finally, it is presented an overview of some worldwide cities’ trolleybus bus experiences, with emphasis to the main operational and environmental drivers.