In 1993, as a joint effort of Eesti Energia and Foster Wheeler a pilot test program was established to determine key parameters for combustion of Estonian oil shale in fluidized-bed boilers. It was concluded that combustion of Estonian oil shale in a circulating fluidized-bed boiler can be successful if the following key points have been understood and taken into account in boiler design: attrition of the fuel and ash; fouling of convection heat surfaces; chlorine corrosion at high temperatures. This paper tells how the key problems above were solved and implemented in the new fluid-bed combustion boiler design for Narva power plants.