A first housing 2 includes a first part 4 and a second part 6, both of which comprise PEEK-OPTIMA (Trade Mark) polymer. The two parts 4, 6 are arranged to cooperate so that they can be laser welded and define a hermetically sealed housing 2 which may house electronic (and/or other) components and maintain them in a dry, sealed environment. The housing and contained electronic components may define an implantable device, for example a heart pacemaker, for long term implantation in a human body.