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 1.3-m telescopes, one covering the northern declination bands and one covering the southern declination bands, were designed specifically for 2MASS to provide all-sky uniformity. 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 first public release of 2MASS data will occur in the late fall of 1998, with a large incremental release scheduled for the spring of 1999.

The point source sensitivity limits (10) are 15.8 (0.8 mJy), 15.1 (1.0 mJy) & 14.3 (1.3 mJy) mag at J,H, Ks, respectively. The extended source sensitivity (10) is ~1 mag fainter than the point source limits, or 14.7 (2.1 mJy), 13.9 (3.0 mJy) & 13.1 (4.0 mJy) mag at J,H, Ks, respectively. Given the approximate angular resolution of the image data, ~2", and the detector sensitivity, the 2MASS survey is well adapted to detection of resolved galaxies out to redshifts >30,000 km/s, as well as compact and diffuse galactic objects. The 2MASS survey is expected to detect over 100 million stars and >1 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 studies with the 2MASS extended source catalog.

The original scientific objectives conceived for the extended source portion of the 2MASS data include large scale structure, construction of the infrared Tully-Fisher relation, complete survey of the local group of galaxies, and an unprecedented 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.

The level-1 requirements apply to the galaxy catalog derived from the 2MASS database. The catalog reliability criterion is in particular a difficult goal to achieve, necessitating design and implementation of special algorithms specifically fabricated to perform star-galaxy separation with 2MASS imaging data. The ? section of this paper describes in some depth the algorithms developed to cleanly discriminate between point sources and extended sources. In the ? section we describe some of the key parametric measurements made on extended sources and the crucial operational step of background removal. In section ?? we introduce the extended source catalog and how it is generated from the 2MASS database, and finally in section ?? we present some basic results from the catalog, including galaxy colors, source counts, completeness and reliability in representative swaths of the sky. We will present more detailed scientific results for field galaxies and galaxy clusters in a future paper.