Meshes - Work only with good topology

A good mesh topology consists of all polygons having only three or four sides in the model (not more). Additionally, edge loops should flow in an ordered, regular way along the contours of the model, defining its shape and form:

On import, Unity automatically converts any NGons (polygons with more than four sides) into triangles, if the mesh has any. However, it's better to build meshes without NGons as opposed to relying on Unity's automated methods. Not only does this cultivate good habits at the modeling phase, it also avoids any automatic and unpredictable re-topology of the mesh, which affects how it's shaded and animated.