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The assembly of the Milky Way bulge is an old topic in astronomy, one now ina period of renewed and rapid development. The dominant scenario for bulgeformation is that of the Milky Way as a nearly pure disk galaxy, with the innerdisk having formed a bar and buckled. This can potentially explain virtuallyall bulge stars with [Fe/H] <~ -1.0, comprising 95% of the stellar population.The evidence is the incredible success in N-body models of this type in makingmeaningful predictions, such as the rotation curve and velocity dispersionmeasured from radial velocities, and the spatial morphologies of thepeanut/X-shape and the long bar. The classical bulge scenario remains viablefor stars with [Fe/H] <~ -1.0 and potentially a minority of the other stars. Aclassical bulge is expected from Lambda-CDM cosmological simulations, canaccentuate the properties of an existing bar in a hybrid system, and is mostconsistent with the bulge abundance trends such as [Mg/Fe], which are elevatedrelative to both the thin and thick disks. Finally, the clumpy-galaxy scenariois considered, as it is the correct description of most Milky Way precursorsgiven observations of high-redshift galaxies. Simulations predict that thesestar-forming clumps will sometimes migrate to the centres of galaxies wherethey may form a bulge, and galaxies often include a bulge clump as well. Theywill possibly form a bar with properties consistent with those of the MilkyWay, such as the exponential profile and metallicity gradient. Given therelative successes of these scenarios, the Milky Way bulge is plausibly ofcomposite origin, with a classical bulge and/or inner halo numerically dominantfor stars with [Fe/H] <~ -1.0, a buckling thick disk for stars with -1.0 <~[Fe/H] <~ -0.50 perhaps descended from the clumpy galaxy phase, and a bucklingthin disk for stars with [Fe/H] >~ -0.50.

Was the Milky Way Bulge Formed From The Buckling Disk Instability, Hierarchical Collapse, Accretion of Clumps, or All of the Above?