A method of controlling a combustion system of a gas turbine engine, the gas turbine engine having: a compressor section with an inlet; a combustor with a primary combustion zone, of which a condition in the primary combustion zone is defined by a primary zone control parameter; a turbine section having an inlet region; the method comprising: measuring, inferring and/or calculating compressor inlet air temperature, controlling the primary zone control parameter (PZCP) to be substantially constant value over a range of values of compressor inlet air temperature, determining a determined turbine entry temperature at the turbine inlet as a function of compressor inlet temperature T1 for a predetermined value of the primary zone control parameter (PZCP), and controlling one or more engine operating parameters to a value, or range of values, determined as a function of compressor inlet air temperature and/or the determined turbine entry temperature; so that actual primary zone control parameter (PZCP) in the combustor is controlled to be substantially equal to the predetermined value of the primary zone control parameter (PZCP); and so the determined turbine entry temperature is within a predetermined range of values. a compressor section with an inlet; a combustor with a primary combustion zone, of which a condition in the primary combustion zone is defined by a primary zone control parameter; a turbine section having an inlet region; the method comprising: measuring, inferring and/or calculating compressor inlet air temperature, controlling the primary zone control parameter (PZCP) to be substantially constant value over a range of values of compressor inlet air temperature, determining a determined turbine entry temperature at the turbine inlet as a function of compressor inlet temperature T1 for a predetermined value of the primary zone control parameter (PZCP), and controlling one or more engine operating parameters to a value, or range of values, determined as a function of compressor inlet air temperature and/or the determined turbine entry temperature; so that actual primary zone control parameter (PZCP) in the combustor is controlled to be substantially equal to the predetermined value of the primary zone control parameter (PZCP); and so the determined turbine entry temperature is within a predetermined range of values. measuring, inferring and/or calculating compressor inlet air temperature, controlling the primary zone control parameter (PZCP) to be substantially constant value over a range of values of compressor inlet air temperature, determining a determined turbine entry temperature at the turbine inlet as a function of compressor inlet temperature T1 for a predetermined value of the primary zone control parameter (PZCP), and controlling one or more engine operating parameters to a value, or range of values, determined as a function of compressor inlet air temperature and/or the determined turbine entry temperature; so that actual primary zone control parameter (PZCP) in the combustor is controlled to be substantially equal to the predetermined value of the primary zone control parameter (PZCP); and so the determined turbine entry temperature is within a predetermined range of values. so that actual primary zone control parameter (PZCP) in the combustor is controlled to be substantially equal to the predetermined value of the primary zone control parameter (PZCP); and so the determined turbine entry temperature is within a predetermined range of values.