The Two Micron All Sky Survey (hereafter, 2MASS) is a ground-based, all-sky survey that utilizes the near-infrared band windows of J(1.25 m), H(1.65 m) and Ks (2.17 m). Conceived over a decade ago (cf. Kleinmann et al. 1994), the project has evolved from an extensive prototype engineering phase (cf. Beichman et al. 1998) to the current operational phase in which survey data has been acquired and accumulating since the spring of 1997 (cf. Skrutskie et al. 1997?). Two dedicated telescopes, one covering the northern declination bands and one covering the southern declination bands, were designed specifically for 2MASS. The data acquisition operations are expected to continue up to 2001 when the sky will be have been covered >98% with satisfactory photometric precision and uniformity. The point source sensitivity limits (10) are 15.8 (?? mJy), 15.1 (?? mJy) & 14.3 (?? mJy) mag at J,H, Ks, respectively. The approximate angular resolution is 2", so the 2MASS survey is well adapted to detection of galaxies and galactic extended sources. The extended source sensitivity (10) is ~1 mag fainter than the point source limits, or 14.7 (?? mJy), 13.9 (?? mJy) & 13.1 (?? mJy) mag at J,H, Ks, respectively. The 2MASS survey is expected to detect over 100 million stas and 2 million galaxies (cf. Chester & Jarrett, 1998). This paper will focus upon the detection/identification and characterization of 2MASS extended sources. Future papers will focus more on specific scientific results with the 2MASS extended source catalog.

The original scientific objectives conceived for the extended source portion of the 2MASS data include large scale structure, constructoin of the infrared Tully-Fisher relation, complete survey of the local group of galaxies, and an unprecidented census of galaxies located behind the plane of the Milky Way, often referred to as the "zone of avoidance". As such, survey requirements were established in order to satisfactorily achieve these science goals. In addition to the sensitivity limits given above, the extended source Level-1 Specifications include >90% completeness and 99% reliability for most of the sky (free of stellar confusion). There are no set requirements for observations deep in the galactic plane.