Heusler alloys are of great interest for their potential use in spintronics. However, their great promise theoretically has yet to be realized experimentally. The discrepancy is expected to be due to structural disorder. In this work, we introduce a new approach of designing materials to control structure-property relationships in Heusler alloys. Promising alloys are first screened and investigated theoretically, and then bulk properties are determined experimentally. Our findings shed light on tuning the electronic and magnetic properties to obtain robust half-metallic and spin-gapless semiconducting behavior. Full Heulser alloys (X2YZ) exhibit exceptional tunability of properties depending upon the elements occupying X and/or Y site. We performed a series of systematic investigations to explore this tunability further: using low valence transition metal atoms (Sc, Ti, V, Cr, Mn, Fe, Co) as a quaternary element on the X site, using low valence transition metals on the Y site in Co2−xYxFeZ (0 ≤ x ≤ 1) for example, and using main group elements (Z = Ge, Si, Ga, Al) on the Z site in Co- and Fe-based ternary Heusler alloys Co2FeZ. In our first example, we used Cr and V in place of Co in disordered Co2FeGe. We found Cr and V not only stabilize disordered Co2FeGe, but also tune the electronic and magnetic properties, making Cr and V ideal dopants. The next system was Co2-xYxFeSi (Y = Sc, Ti, V, Cr, Mn, or Fe, 0 ≤ x ≤ 1), which eventually led to a global overview of the structural, magnetic, electronic, and mechanical properties of promising candidate materials. Based on this, we synthesized Co2-xYxFe(Ga,Al) (Y = Ti, V, Cr, Mn, or Fe, x = 0.50) and found Co1.50Mn0.50Fe(Ga,Al) showed spin-gapless semiconducting behavior. Other alloys in the series also show interesting tunable properties. Inspired by earlier work showing a hexagonal Heusler analogue may retain half-metallic character and show high magneto-crystalline anisotropy, we investigated Fe-based Heusler compounds Fe3-xYxGe (Y = V, or Cr, 0 ≤ x ≤ 1) with a hexagonal DO19 structure. This system shows high magnetic moments and magneto-crystalline anisotropy, which takes another step toward practical applications in perpendicular media and CPP-GMR.