The development of silicon microcomponents and microsystems, being an outgrowth of the processing techniques commonly used for the fabrication of integrated circuits, was constrained to mostly planar dimensions. This was because optical lithography and most photoresists were developed for this specific purpose. In many applications, however, the components require that the vertical dimensions be larger than those obtained by traditional optical lithography methods. This led to the development of high aspect ratio processing in which the vertical dimension can be large compared to the lateral dimensions. While there is no apparent agreed-upon definition of high aspect ratio, the processes are generally characterized by x-ray lithography and/or micromechanical machining methods, with postprocessing and replication methods suitable for the dimensions involved. While there have been many accomplishments toward making these processes as economically viable as integrated circuit processes, much work remains to be done. The following sections describe the high aspect ratio processes and replication techniques. The work is a compilation of efforts worldwide and is meant to provide an overview of the processes and techniques.