A system for virtual interaction, comprising two or more portable electronic devices, each device comprising: one or more accelerometers; coordinate referencing means operable to define a coordinate system common to the portable electronic devices with respect to a physically defined reference position wherein the portable electronic devices are physically placed adjacent to one another to define said common reference position; position estimation means operable to detect the physical position of its respective portable electronic device with respect to the reference position based upon a cumulative change in displacement from the reference position as detected using the one or more accelerometers; virtual environment generation means (100, 220) operable to generate a virtual environment for display upon a display of the device; and communication means operable to transmit positional data using the common coordinate system from that portable electronic device to another portable electronic device; and in which the virtual environment is shared in common between the portable electronic devices, the virtual environment using the common co-ordinate system within which each portable electronic device defines a virtual position for itself responsive to its physical position with respect to the reference position, wherein at least one of the portable electronic device comprises a video camera operable to capture a video image and is operable to detect an error in the correspondence between the real physical relative position of another portable electronic device and virtual relative position of the other portable electronic device within the common coordinate system, by analysing a captured video image to detect whether the other portable electronic device lies within a region of the video image predicted to encompass the real physical relative position of the other portable electronic device based upon its virtual relative position within the common coordinate system. one or more accelerometers; coordinate referencing means operable to define a coordinate system common to the portable electronic devices with respect to a physically defined reference position wherein the portable electronic devices are physically placed adjacent to one another to define said common reference position; position estimation means operable to detect the physical position of its respective portable electronic device with respect to the reference position based upon a cumulative change in displacement from the reference position as detected using the one or more accelerometers; virtual environment generation means (100, 220) operable to generate a virtual environment for display upon a display of the device; and communication means operable to transmit positional data using the common coordinate system from that portable electronic device to another portable electronic device; and in which the virtual environment is shared in common between the portable electronic devices, the virtual environment using the common co-ordinate system within which each portable electronic device defines a virtual position for itself responsive to its physical position with respect to the reference position,