The implementation of market mechanisms, different support schemes for renewable energy resources and large-scale integration of intermittent generation in energy production process have a negative effect on running conditions of fossil fuel thermal power plants (TPPs) and their future. The operation of fossil fuel TPPs is shifted from based load operation to cycling, which is adverse from technical, economic and environmental point of view. Moreover, the thermal power plants are partly or not at all adapted to new running conditions. This leads to the efficiency decrease and more frequent trips and outages of power plants. Thus, the flexibility level of fossil fuel TPPs should be increased to adapt their operation to new running conditions and increase their efficiency, flexibility and profitable operation as well as prevent from mothballing. There are different measures to increase the flexibility of thermal power plants. The Doctoral Thesis provides the mathematical description of transient modes of the combined cycle gas turbine (CCGT) power plants. The EM&OM (Evaluation Model and Optimisation Model) approach was developed to optimise its operation by shifting start-up “backward” and shutdown “forward”. The general algorithm was developed to provide the technical and economic justification of technologies, which are aimed at increasing the flexibility level of fossil fuel TPP. The algorithm was adapted to three technologies: air cooling, electric boiler and heat storage tank. The developed measures were approbated on the example of Riga TPP-2 plant in Latvian site conditions. The presented measures can be adapted to other types of thermal power plants. The obtained results can also be used by JSC Latvenergo to decide about the necessity to increase the flexibility level of Riga TPP-2 and by researchers of Riga Technical University to use them as input data for the developed optimisation programme of power plants.