Astrophotonics is the application of versatile photonic technologies to channel, manipulate, and disperse guided light from one or more telescopes to achieve scientific objectives in astronomy in an efficient and cost-effective way. The field of astrophotonics spans a wide range of technologies, including collecting astronomical light into guided channels (fibers/waveguides), manipulating the transport and reconfiguration of the light, and filtering/dispersing/combining the guided light. A combination of one or more of these functionalities has led to a wide spectrum of astrophotonic instruments. The developments and demands from the telecommunication industry have driven a major boost in photonic technology and vice-versa in the last decades. The photonic platform of guided light in fibers and waveguides has opened the doors to next-generation instrumentation for both ground- and space-based telescopes in optical and near/mid-IR bands, particularly for the upcoming large-aperture telescopes. The large telescopes are pushing the limits of adaptive optics to reach close to a near-diffraction-limited performance. The photonic devices are ideally suited for capturing this AO-corrected light and enabling new and exciting science such as characterizing exoplanet atmospheres. The ongoing growth of photonics industry and astrophotonics displays a strong parallel with the development of radio communication and radio astronomy, where each positively influenced the other.The paper introduces the photonic lantern, an astrophotonic device that allow for a low-loss transformation of a multimode waveguide into a discrete number of single-mode waveguides and vice versa, thus enabling the use of single-mode photonic technologies in multimode systems. The paper discusses the theory and function of the photonic lantern, categories and manufacture of the device, and several applications in astronomical observations.How the uncoupled SM waveguide modes evolve through an adiabatic transition to become the modes of the MM waveguide can be described by analogy with the Kronig-Penney model for the interaction of electrons in a periodic potential well. In order to understand how a set of identical modes evolve into an equal number of non-degenerate modes, the entire taper transition of the photonic lantern must be modeled. At large lantern diameters the modes of the single-mode cores are strongly confined, and thus do not couple and remain near-degenerate. As the lantern diameter decreases, the modal fields expand, such that interaction between the single-mode cores increases. The resulting coupling leads the formation of non-degenerate supermodes that form from linear superposition of these original modes.While PLs come in a wide array of port counts and geometries, they can be largely classified into three groups. In what we call the “standard” PL, embedded cores are uniform in structure and refractive index. At the other extreme, “mode-selective” PLs use differing single-mode core radii or index contrasts, so that each fiber mode at the FMF-like lantern entrance routes to a distinct output por. Lastly, we term lanterns that operate between these two extremes “hybrid lanterns.” These lanterns have one core mismatched from the rest, thereby funnelling light from the fundamental fiber mode into a single output port while mixing the remaining light in the rest of the ports.Here we outline the five types of photonic lanterns that have been reported to date.1) Compact astronomical spectral filters. FBGs operate at the diffraction-limit which makes them difficult and inefficient to couple light to from either a seeing-limited telescope (a telescope that does not use Adaptive Optics (AO) to correct for the atmospheric turbulence) and/or a telescope which has a low performance AO system. The PLs described in the previous section form the ideal solution to collect light from such telescopes efficiently and exploit FBGs for spectral filtering.2) Efficient fiber scrambling. Due to imperfections in the fiber, light couples between modes and excites new modes as it propagates along the fiber leading to an unstable output beam that varies with wavelength and time. This effect can also be confused as a barycenter shift in the measured spectral line and is known as modal noise. To combat this, fibers are generally agitated. MCFs in combination with PLs offer a unique solution to these issues. Indeed, it was realized early-on that a back-to-back PL built from a MCF could offer mode scrambling properties.3) Wavefront sensing. The photonic lantern WFS (PL-WFS) represents a type of WFS, which addresses several of the limitations of current adaptive optics systems. Placing the WFS at the focal plane, rather than at a non-common pupil plane, has been long desired in adaptive optics as it eliminates non-common path error and is sensitive to wavefront errors not visible in the pupil plane (such as island modes).Photonic lanterns allow for a multimode core to be converted to several single-mode cores. Owing to the multimode input, a PL is ideally suited to efficiently couple a low-quality beam (as a result of the atmospheric turbulence) from the focus of a telescope while delivering a diffraction-limited beam to an instrument.In conclusion, the photonic lantern is a versatile and powerful concept, allowing the transformation of an optical multimode system into a single-mode one and enabling the use of single-mode-based photonic technologies in multimode systems for the first time. Photonic lanterns increase the functionality and possible applications of few-modal devices and systems. These mode convertors offer the possibility of improving the light collecting ability while keeping and opening new photonic functionalities in astrophotonics and astronomical instruments.