Crystal engineering studies molecular solid-state structures to understand the intermolecular interactions. There are two main concepts studied in crystal engineering which are hydrogen bonding and coordination complexation. These are understood with key concepts such as the supramolecular synthon and the secondary building unit.
Polymorphism is where the same chemical compound exists in different crystal forms. In the intial stages of crystal engineering, polymorphism was not understood and was incompletely studied. Today, it is one of the most exciting branches of the subject partly because polymorphic forms of drugs may be entitled to independent patent protection if they show new and improved properties over the known crystal forms.