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NASA's Interface Region Imaging Spectrograph (IRIS) space mission will studyhow the solar atmosphere is energized. IRIS contains an imaging spectrographthat covers the Mg II h&k lines as well as a slit-jaw imager centered at Mg IIk. Understanding the observations will require forward modeling of Mg II h&kline formation from 3D radiation-MHD models. This paper is the first in aseries where we undertake this forward modeling. We discuss the atomic physicspertinent to h&k line formation, present a quintessential model atom that canbe used in radiative transfer computations and discuss the effect of partialredistribution (PRD) and 3D radiative transfer on the emergent line profiles.We conclude that Mg II h&k can be modeled accurately with a 4-level pluscontinuum Mg II model atom. Ideally radiative transfer computations should bedone in 3D including PRD effects. In practice this is currently not possible. Areasonable compromise is to use 1D PRD computations to model the line profileup to and including the central emission peaks, and use 3D transfer assumingcomplete redistribution to model the central depression.

The formation of IRIS diagnostics I. A quintessential model atom of Mg II and general formation properties of the Mg II h&k lines