Microwave container and method of using same

A container for holding a body (160) of material such as foodstuff to be heated in a microwave oven, including a dielectric lid (161) and/or other dielectric wall structure with at least two contiguous dielectric wall portions (166,168) of respectively different electrical thickness for co-operatively modifying the microwave electric field patterns that would obtain in the body during such heating if the lid and/or other dielectric wall structure were of uniform electrical thickness or were absent. At least one of the portions, preferably the portion of higher electrical thickness, may be so constituted as to undergo a change in dielectric constant during the microwave heating, thereby to alter the heating distribution in the body as heating proceeds.