A method of de-icing a wind turbine blade, the method comprising the steps of: generating heated air using heating means provided in the root portion of the blade; and continuously circulating the heated air around the interior of the blade through at least a portion of two or more longitudinal blade cavities (24, 26, 28) defined within the blade, the circulating step including: channelling the heated air from an outlet of the heating means at least part way through a first longitudinal blade cavity, wherein the first longitudinal blade cavity is a trailing edge cavity defined between one or more longitudinal webs and the trailing edge of the blade, towards the tip end of the blade; at a position along the length of the blade, diverting a first portion of the heated air from the trailing edge cavity into a central cavity and diverting a second portion of the heated air from the trailing edge cavity into a leading edge cavity; and channelling the diverted air at least part way through the leading edge cavity and the central cavity back to an inlet of the heating means, wherein the heated air is circulated through the trailing edge cavity, the central cavity defined between longitudinal webs extending through the interior of the blade, and the leading edge cavity defined between one or more longitudinal webs and the leading edge of the blade. generating heated air using heating means provided in the root portion of the blade; and continuously circulating the heated air around the interior of the blade through at least a portion of two or more longitudinal blade cavities (24, 26, 28) defined within the blade, the circulating step including: channelling the heated air from an outlet of the heating means at least part way through a first longitudinal blade cavity, wherein the first longitudinal blade cavity is a trailing edge cavity defined between one or more longitudinal webs and the trailing edge of the blade, towards the tip end of the blade; at a position along the length of the blade, diverting a first portion of the heated air from the trailing edge cavity into a central cavity and diverting a second portion of the heated air from the trailing edge cavity into a leading edge cavity; and channelling the diverted air at least part way through the leading edge cavity and the central cavity back to an inlet of the heating means, wherein the heated air is circulated through the trailing edge cavity, the central cavity defined between longitudinal webs extending through the interior of the blade, and the leading edge cavity defined between one or more longitudinal webs and the leading edge of the blade. channelling the heated air from an outlet of the heating means at least part way through a first longitudinal blade cavity, wherein the first longitudinal blade cavity is a trailing edge cavity defined between one or more longitudinal webs and the trailing edge of the blade, towards the tip end of the blade; at a position along the length of the blade, diverting a first portion of the heated air from the trailing edge cavity into a central cavity and diverting a second portion of the heated air from the trailing edge cavity into a leading edge cavity; and channelling the diverted air at least part way through the leading edge cavity and the central cavity back to an inlet of the heating means, wherein the heated air is circulated through the trailing edge cavity, the central cavity defined between longitudinal webs extending through the interior of the blade, and the leading edge cavity defined between one or more longitudinal webs and the leading edge of the blade.