Here we will present all-colloidal lasers constructed by incorporating nanocrystal emitters as the optical gain media in fully colloidal cavities. As an extreme case of solution-processed tightly-confined quasi-2D colloids, we will also show that the atomically flat nanocrystals, alternatively known as colloidal quantum wells, uniquely offer record high optical gain coefficients and ultra-low amplified spontaneous emission thresholds.