A computer-implemented method of tessellating a patch to generate tessellated geometry data representing the tessellated patch, the method comprising: processing received geometry data representing a patch to identify tessellation factors of the patch; determining, based on the identified tessellation factors of the patch, tessellation instances to be used in tessellating the patch, wherein the tessellation instances are portions of tessellation work, and wherein each of the tessellation instances, determined for the patch, is associated with a portion of tessellated geometry that will be generated when the patch is tessellated so that the tessellated geometry associated with all of the tessellation instances for the patch collectively define the tessellated geometry data for the patch; and allocating the tessellation instances amongst a plurality of tessellation pipelines (230-1, 230-2, 230-3) that operate in parallel, wherein a respective set of one or more of the tessellation instances is allocated to each of the tessellation pipelines (230-1, 230-2, 230-3), and wherein each of the tessellation pipelines (230-1, 230-2, 230-3) generates the tessellated geometry data associated with the respective allocated set of one or more of the tessellation instances, characterised in that: said determining tessellation instances to be used in tessellating the patch comprises determining a number of tessellation instances J to be used in tessellating the patch by: determining a number of vertices N verts that are to be generated for the patch during tessellation based on the determined tessellation factors for the patch; and dividing the number of vertices N verts by a pre-determined number L, such that J = Roundup N verts L, wherein the pre-determined number L is determined based on the amount of memory storage available to store vertex data generated by each of the tessellation pipelines. processing received geometry data representing a patch to identify tessellation factors of the patch; determining, based on the identified tessellation factors of the patch, tessellation instances to be used in tessellating the patch, wherein the tessellation instances are portions of tessellation work, and wherein each of the tessellation instances, determined for the patch, is associated with a portion of tessellated geometry that will be generated when the patch is tessellated so that the tessellated geometry associated with all of the tessellation instances for the patch collectively define the tessellated geometry data for the patch; and allocating the tessellation instances amongst a plurality of tessellation pipelines (230-1, 230-2, 230-3) that operate in parallel, wherein a respective set of one or more of the tessellation instances is allocated to each of the tessellation pipelines (230-1, 230-2, 230-3), and wherein each of the tessellation pipelines (230-1, 230-2, 230-3) generates the tessellated geometry data associated with the respective allocated set of one or more of the tessellation instances, characterised in that: said determining tessellation instances to be used in tessellating the patch comprises determining a number of tessellation instances J to be used in tessellating the patch by: determining a number of vertices N verts that are to be generated for the patch during tessellation based on the determined tessellation factors for the patch; and dividing the number of vertices N verts by a pre-determined number L, such that J = Roundup N verts L, wherein the pre-determined number L is determined based on the amount of memory storage available to store vertex data generated by each of the tessellation pipelines.