All-inorganic colloidal cesium lead halide perovskite quantum dots (CsPbX3 , X = Cl, Br, I) are revealed to be a new class of favorable optical gain materials, which show -combined merits of both colloidal quantum dots and halide perovskites. Low-threshold and -ultrastable stimulated emission is -demonstrated under atmospheric conditions with wavelength tunability across the whole -visible spectrum via either size or composition control.