A large number of applications, e.g., sensors, microelectromechanical systems (MEMS) or high-frequency (HF) antennas, rely on metallic components with on the one hand micrometer-sized features and on the other hand extensions in the millimeter range [
1]. These requirements are difficult to meet with traditional fabrication technologies since they are somewhere in the gray zone between subtractive microstructuring or selective laser melting and electron beam lithography or electron/ion beam induced deposition. Not surprisingly, some additive manufacturing technologies have been developed that try to occupy this zone: direct ink writing (DIW, extrusion of metal particle inks, [
2]), electrohydrodynamic printing (EHD, electrohydrodynamic ejection of droplets [
3]), laser-assisted electrophoretic deposition (LAED, electrophoretic deposition of nanoparticles and laser trapping [
4]), laser-induced forward transfer (LIFT, laser-induced ejection of liquid metal droplets [
5]), meniscus-confined electroplating (MCE, electroplating from a metal salt solution only at the meniscus formed between pipette and substrate [
6]), electroplating of locally dispensed ions in liquid (ELD, electroplating in the interaction region of a dispersed electrolyte and a supporting electrolyte [
7]), liquid metal-based direct writing (LMDW, extrusion of a liquid metal from a nozzle while the liquid metal column is stabilized by its oxide skin [
8,
9]), and direct laser writing (DLW, covered here, for reviews of all these methods see [
10,
11]). Among these, DLW as a rapid-prototyping fabrication technique offers the greatest versatility, since—a requirement for on-chip structuring capabilities—it does not rely on specific, e.g., conductive, substrates (contrary to EHD, LAED, MCE, and ELD) [
12,
13]. Furthermore, DLW enables merely unlimited complex structure geometries outperforming most other technologies including LIFT, LMDW, and DIW in this respect.
A disadvantage of DLW is, however, the rather limited range of photosensitive materials available, since up to now most research has been devoted to polymer-based photoresists [
14]. In these resists via multiphoton absorption of a tightly focused laser beam, a liquid polymer selectively hardens by cross-linking reactions (negative-tone resists) or a solid polymer selectively turns soluble by, e.g., carboxylic acid generation (positive-tone resists). To the contrary, DLW of metallic structures—first reported in 2006 [
15]—is based on photoreduction of dissolved metal ions to neutral metal atoms and subsequent nucleation, growth, and aggregation (introduced in
Section 2, for a detailed review of the involved processes see [
16]). Since then, a number of groups have fabricated metallic microstructures using DLW either directly [
17,
18,
19,
20,
21] or indirectly via post-illumination metallization [
22,
23,
24,
25,
26,
27,
28,
29,
30,
31,
32,
33] such as electroless or galvanic growth or plasma sputtering. While indirect processes yield outstanding structure quality, they reduce the versatility of the structuring process since mostly these post-processes are not compatible with on-chip structuring (these processes, e.g., require conductive substrates, do not easily allow for fabrication only on selected parts of the chip or need high temperature treatment) and do not easily allow for fully disconnected structures embedded within a matrix. Therefore, in
Section 4 and
Section 5 of this review we focus on metallic components with proven functionality directly produced via DLW (introduced in
Section 2). The components presented there are all made of noble metals since non-noble metals are not easily produced in a direct laser writing approach due to their unfavourable reduction potential. Accordingly, some post-illumination metallized structures made of non-noble metals are also presented in
Section 3. In total, we show that metal direct laser writing (MDLW) of planar structures has reached the level of maturity required for applications while direct fabrication of functional three-dimensional (3D) metallic microcomponents is waiting in the wings.