The purpose of this paper and its sequel is to introduce and develop a theory of toric stacks which encompasses and extends several notions of toric stacks defined in the literature, as well as classical toric varieties.In this paper, we define a toric stack as the stack quotient of a toric variety by a subgroup of its torus (we also define a generically stacky version). Any toric stack arises from a combinatorial gadget called a stacky fan. We develop a dictionary between the combinatorics of stacky fans and the geometry of toric stacks, stressing stacky phenomena such as canonical stacks and good moduli space morphisms.We also show that smooth toric stacks carry a moduli interpretation extending the usual moduli interpretations of P n and [A 1 /G m ]. Indeed, smooth toric stacks precisely solve moduli problems specified by (generalized) effective Cartier divisors with given linear relations and given intersection relations.Smooth toric stacks therefore form a natural closure to the class of moduli problems introduced for smooth toric varieties and smooth toric DM stacks in papers by Cox and Perroni, respectively.We include a plethora of examples to illustrate the general theory. We hope that this theory of toric stacks can serve as a companion to an introduction to stacks, in much the same way that toric varieties can serve as a companion to an introduction to schemes.License or copyright restrictions may apply to redistribution; see http://www.ams.org/journal-terms-of-use TORIC STACKS I: THE THEORY OF STACKY FANS 1035This definition encompasses and extends the three kinds of toric stacks listed above:• Taking G to be trivial, we see that any toric variety X is a toric stack. • Smooth toric Deligne-Mumford stacks in the sense of [BCS05, FMN10, Iwa09] are smooth non-strict toric stacks which happen to be separated and Deligne-Mumford. See Remarks 2.17 and 2.18. • Toric stacks in the sense of [Laf02] are toric stacks that have a dense open point (i.e. toric stacks for which G = T 0 ). • A toric Artin stack in the sense of [Sat12] is a smooth non-strict toric stack which has finite generic stabilizer and which has a toric variety of the same dimension as a good moduli space. See Sections 4 and 6. • Toric stacks in the sense of [Tyo12] are toric stacks as well. This follows from the main theorem of [GS11b], stated below. See [GS11b, Remark 6.2]for more details.In this notation, the stack in Example 2.7 would be denoted [A 2 / ( 1 1 ) μ 2 ].Example 2.9. Again we have that X Σ = A 2 . This time β * = ( 1 0 ) : Z → Z 2 , which induces the homomorphism G 2 m → G m given by (s, t) → s. Therefore,We then have that X Σ,β = [(A 2 {(0, 0)})/ ( 1 1 ) G m ] = P 1 . Warning 2.11. Examples 2.6 and 2.10 show that non-isomorphic stacky fans (see Definition 3.2) can give rise to isomorphic toric stacks. The two presentations [(A 2 {(0, 0)})/ ( 1 1 ) G m ] and [P 1 /{e}] of the same toric stack are produced by different stacky fans. In Theorem B.3, we determine when different stacky fans give rise to the same toric stack.Example 2....