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 (216, 500) attached to the interior side of the cover, the optical sensor subsystem (216, 500) comprising: a substrate (232, 504); a set of light emitters (400, 402, 506, 908) adjacent a central portion of the cover and configured to emit light through the cover; and a set of light receivers (404, 508) substantially surrounding the set of light emitters (400, 402, 506, 908) 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 (406, 408, 516, 518) attached to the interior side of the cover and to the substrate (232, 504), thereby attaching the substrate (232, 504) to the cover; a lens attached to the interior side of the cover and positioned between the set of light emitters (400, 402, 506, 908) and the cover; a light filter (210, 522) attached to the interior side of the cover and positioned between at least one of the light receivers (404, 508) and the cover; and a magnet (220, 528) attached to the substrate (232, 504); 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 (216, 500) attached to the interior side of the cover, the optical sensor subsystem (216, 500) comprising: a substrate (232, 504); a set of light emitters (400, 402, 506, 908) adjacent a central portion of the cover and configured to emit light through the cover; and a set of light receivers (404, 508) substantially surrounding the set of light emitters (400, 402, 506, 908) and positioned to receive a reflected portion of the light and through the cover; a substrate (232, 504); a set of light emitters (400, 402, 506, 908) adjacent a central portion of the cover and configured to emit light through the cover; and a set of light receivers (404, 508) substantially surrounding the set of light emitters (400, 402, 506, 908) 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 (406, 408, 516, 518) attached to the interior side of the cover and to the substrate (232, 504), thereby attaching the substrate (232, 504) to the cover; a lens attached to the interior side of the cover and positioned between the set of light emitters (400, 402, 506, 908) and the cover; a light filter (210, 522) attached to the interior side of the cover and positioned between at least one of the light receivers (404, 508) and the cover; and a magnet (220, 528) attached to the substrate (232, 504); and a processor configured to determine a biological parameter using the reflected portion of the light. a set of light-blocking walls (406, 408, 516, 518) attached to the interior side of the cover and to the substrate (232, 504), thereby attaching the substrate (232, 504) to the cover; a lens attached to the interior side of the cover and positioned between the set of light emitters (400, 402, 506, 908) and the cover; a light filter (210, 522) attached to the interior side of the cover and positioned between at least one of the light receivers (404, 508) and the cover; and a magnet (220, 528) attached to the substrate (232, 504); and a processor configured to determine a biological parameter using the reflected portion of the light.