This disclosure relates to the packaging of microwave integrated circuits (MICs) whereby an MIC is hermetically sealed within an enclosure comprising a first plate of dielectric material which carries the circuit to be enclosed, a wall of dielectric material sealed to the surface of the first plate surrounding the circuit, and a second plate of dielectric material providing a lid sealed over the wall to complete the enclosure. The first plate carries planar conductors which define at least one microwave transmission line, e.g. microstrip, extending across the wall into the enclosure from outside to provide direct microwave coupling to the enclosed circuit and thereby obviating the usual need for transitions to and from coaxial cable.Part or all of the enclosed circuit may be contained within a recess provided by forming an aperture in the first plate and sealing a third plate of dielectric material across the aperture from below.The disclosure describes various techniques for reducing losses where the transmission line propagation paths traverse the wall on the surface of the first plate. The entire package may be fabricated using materials and techniques compatible with those used in fabricating MIC's, for example thick film.