Introduction: Why and How to Probe the Zone of Avoidance ?

The motivation and major ways for probing the Zone of Avoidance (ZOA) are reviewed. Galaxies hidden behind the ZOA may have important implications for the internal dynamics of the Local Group, for the origin of its motion relative to the Microwave Background, and for the connectivity of the large scale structure. Current direct (observational') methods for exploring the ZOA include eye-balling of plates, source identification in the IRAS data base and pointed and blind-search observations in 21 cm. Interesting regions identified so far include the two crossing points of the Supergalactic Plane by the Galactic Plane (at Galactic longitude $l \sim 135^o$, near Perseus-Pisces, and $l \sim 315^o$, near the Great Attractor), the Puppis cluster (at $l \sim 240^o, cz \sim 1500$ km/sec) and the Ophiuchus cluster (at $l \sim 0^o, cz \sim 8400$ km/sec). New promising wavelengths are the $2 \mu$ and the X-ray band. Indirect (theoretical’) approaches include `Wiener reconstruction’ from incomplete and noisy data, and using the peculiar velocity field as a probe of the mass distribution hidden behind the ZOA. The problem of source confusion at low Galactic latitude can be addressed by novel statistical methods, e.g. Artificial Neural Networks.