A capsule for preparing a beverage comprising a cup-shaped body and a lid; the cup-shaped body having a base and a side wall and the lid being sealed to the cup-shaped body; the capsule being designed for insertion into a beverage preparation machine to permit a pressurised liquid to be flowed through the capsule in order to produce a beverage from interaction with the beverage ingredients; the beverage preparation machine being of the type having an enclosing member adapted to be selectively movable between an open position to permit insertion of the capsule into the beverage preparation machine and a closed position in which the enclosing member sealingly engages the capsule; wherein the side wall comprises: - an annular trough being dimensioned to receive the enclosing member; - a first side wall section extending between the base and the annular trough; and - a second side wall section extending between the annular trough and a rim of the capsule; wherein the side wall is adapted to undergo plastic deformation during closure of the enclosing member; wherein the annular trough is adapted to form a sealing interface with a leading edge of the enclosing member; wherein, prior to insertion, the annular trough comprises an inner wall, an outer wall and a floor; wherein the rim is formed integrally with the cup-shaped body; wherein the rim is formed by a rolled-over portion of the side wall; wherein the cup-shaped body is formed from aluminium or an aluminium alloy; wherein the cup-shaped body is formed from a single integral piece of material; and wherein a thickness of the cup-shaped body is in the range of 80 to 120 microns. the capsule being designed for insertion into a beverage preparation machine to permit a pressurised liquid to be flowed through the capsule in order to produce a beverage from interaction with the beverage ingredients; the beverage preparation machine being of the type having an enclosing member adapted to be selectively movable between an open position to permit insertion of the capsule into the beverage preparation machine and a closed position in which the enclosing member sealingly engages the capsule; wherein the side wall comprises: - an annular trough being dimensioned to receive the enclosing member; - a first side wall section extending between the base and the annular trough; and - a second side wall section extending between the annular trough and a rim of the capsule; - an annular trough being dimensioned to receive the enclosing member; - a first side wall section extending between the base and the annular trough; and - a second side wall section extending between the annular trough and a rim of the capsule; wherein the side wall is adapted to undergo plastic deformation during closure of the enclosing member; wherein the annular trough is adapted to form a sealing interface with a leading edge of the enclosing member; wherein, prior to insertion, the annular trough comprises an inner wall, an outer wall and a floor; wherein the rim is formed integrally with the cup-shaped body; wherein the rim is formed by a rolled-over portion of the side wall; wherein the cup-shaped body is formed from aluminium or an aluminium alloy; wherein the cup-shaped body is formed from a single integral piece of material; and wherein a thickness of the cup-shaped body is in the range of 80 to 120 microns.