A mitral valve prosthesis for replacing a native mitral valve having a native mitral valve orifice, the mitral valve prosthesis comprising a housing component and a valve component; wherein said housing component comprises: a housing body having a housing passage extending therethrough; and a flexible skirt extending about and projecting radially outwardly from a periphery of the housing body for inhibiting blood flow in a first direction between the housing body and a wall of the native mitral valve orifice; said housing body comprising a housing body frame formed of one or more elongate elastic housing body frame elements, and a flexible housing wall fixed to said housing body frame and extending about said housing passage; said housing body being configured to be located in the native mitral valve orifice of a heart; said housing component being configurable between a housing collapsed state for delivery to the native mitral valve orifice via catheter and a housing expanded state to engage structure of the heart to fix said housing body in relation to the native mitral valve orifice; further wherein said valve component is in the form of a stent valve and comprises: an expandable valve body having a valve passage extending therethrough; said valve body being configured to be located wholly within said housing passage of the housing body with said valve passage extending along said housing passage; and one or more flexible valve elements secured to said valve body and extending across said valve passage for blocking blood flow in the first direction through said valve passage whilst allowing blood flow in an opposing second direction through said valve passage; said valve component being configurable between a valve collapsed state for delivery to said housing passage via catheter, when said housing body is in said housing expanded state following delivery to the native mitral valve orifice, and an expanded state to engage said housing component to fix said valve body within said housing passage. wherein said housing component comprises: a housing body having a housing passage extending therethrough; and a flexible skirt extending about and projecting radially outwardly from a periphery of the housing body for inhibiting blood flow in a first direction between the housing body and a wall of the native mitral valve orifice; said housing body comprising a housing body frame formed of one or more elongate elastic housing body frame elements, and a flexible housing wall fixed to said housing body frame and extending about said housing passage; said housing body being configured to be located in the native mitral valve orifice of a heart; said housing component being configurable between a housing collapsed state for delivery to the native mitral valve orifice via catheter and a housing expanded state to engage structure of the heart to fix said housing body in relation to the native mitral valve orifice; a housing body having a housing passage extending therethrough; and a flexible skirt extending about and projecting radially outwardly from a periphery of the housing body for inhibiting blood flow in a first direction between the housing body and a wall of the native mitral valve orifice; said housing body comprising a housing body frame formed of one or more elongate elastic housing body frame elements, and a flexible housing wall fixed to said housing body frame and extending about said housing passage; said housing body being configured to be located in the native mitral valve orifice of a heart; said housing component being configurable between a housing collapsed state for delivery to the native mitral valve orifice via catheter and a housing expanded state to engage structure of the heart to fix said housing body in relation to the native mitral valve orifice; further wherein said valve component is in the form of a stent valve and comprises: an expandable valve body having a valve passage extending therethrough; said valve body being configured to be located wholly within said housing passage of the housing body with said valve passage extending along said housing passage; and one or more flexible valve elements secured to said valve body and extending across said valve passage for blocking blood flow in the first direction through said valve passage whilst allowing blood flow in an opposing second direction through said valve passage; said valve component being configurable between a valve collapsed state for delivery to said housing passage via catheter, when said housing body is in said housing expanded state following delivery to the native mitral valve orifice, and an expanded state to engage said housing component to fix said valve body within said housing passage. an expandable valve body having a valve passage extending therethrough; said valve body being configured to be located wholly within said housing passage of the housing body with said valve passage extending along said housing passage; and one or more flexible valve elements secured to said valve body and extending across said valve passage for blocking blood flow in the first direction through said valve passage whilst allowing blood flow in an opposing second direction through said valve passage; said valve component being configurable between a valve collapsed state for delivery to said housing passage via catheter, when said housing body is in said housing expanded state following delivery to the native mitral valve orifice, and an expanded state to engage said housing component to fix said valve body within said housing passage.