An electronic watch , comprising: a housing comprising a frontside housing member and a backside housing member , the backside housing member defining a first aperture extending through a back wall of the housing ; a cover formed of a unitary piece of a transparent material and positioned in the first aperture, the cover having an exterior side defining a portion of a back surface of the electronic watch and an interior side opposite the exterior side; a set of internal components coupled to the cover and comprising: an optical sensor subsystem attached to the interior side of the cover , the optical sensor subsystem comprising: a substrate ; a set of light emitters adjacent a central portion of the cover and configured to emit light through the cover ; and a set of light receivers substantially surrounding the set of light emitters and positioned to receive a reflected portion of the light and through the cover ; the set of internal components further comprising: a set of light-blocking walls attached to the interior side of the cover and to the substrate , thereby attaching the substrate to the cover ; a lens attached to the interior side of the cover and positioned between the set of light emitters and the cover ; a light filter attached to the interior side of the cover and positioned between at least one of the light receivers and the cover ; and a magnet attached to the substrate ; and a processor configured to determine a biological parameter using the reflected portion of the light. a housing comprising a frontside housing member and a backside housing member , the backside housing member defining a first aperture extending through a back wall of the housing ; a cover formed of a unitary piece of a transparent material and positioned in the first aperture, the cover having an exterior side defining a portion of a back surface of the electronic watch and an interior side opposite the exterior side; a set of internal components coupled to the cover and comprising: an optical sensor subsystem attached to the interior side of the cover , the optical sensor subsystem comprising: a substrate ; a set of light emitters adjacent a central portion of the cover and configured to emit light through the cover ; and a set of light receivers substantially surrounding the set of light emitters and positioned to receive a reflected portion of the light and through the cover ; a substrate ; a set of light emitters adjacent a central portion of the cover and configured to emit light through the cover ; and a set of light receivers substantially surrounding the set of light emitters and positioned to receive a reflected portion of the light and through the cover ; the set of internal components further comprising: a set of light-blocking walls attached to the interior side of the cover and to the substrate , thereby attaching the substrate to the cover ; a lens attached to the interior side of the cover and positioned between the set of light emitters and the cover ; a light filter attached to the interior side of the cover and positioned between at least one of the light receivers and the cover ; and a magnet attached to the substrate ; and a processor configured to determine a biological parameter using the reflected portion of the light. a set of light-blocking walls attached to the interior side of the cover and to the substrate , thereby attaching the substrate to the cover ; a lens attached to the interior side of the cover and positioned between the set of light emitters and the cover ; a light filter attached to the interior side of the cover and positioned between at least one of the light receivers and the cover ; and a magnet attached to the substrate ; and a processor configured to determine a biological parameter using the reflected portion of the light. <EOS>