**Preface to "Multicomponent Pharmaceutical Solids"**

Multicomponent pharmaceutical materials are solids in which at least one component is an active pharmaceutical ingredient (API). This kind of pharmaceutical solid has certainly revolutionized the pharmaceutical industry, proving to be an interesting alternative to the laborious and expensive traditional pipeline drug development. The application of crystal engineering techniques into the design of pharmaceutical salts, co-crystals, and other multicomponent materials has succeeded in modulating the physicochemical, mechanical, and pharmacokinetic properties of drugs, thereby working towards the enhancement of their clinical performance. In addition, environmentally friendly synthetic approaches have also been achieved, in agreement with the new green chemistry principles.

Despite its promising future, the study of pharmaceutical solids still presents significant challenges. For instance, unraveling structure–activity relationships or the preference for supramolecular synthons, and choosing an appropriate co-former and/or solvent selection are still issues that need to be thoroughly studied in order to fully understand, and consequently predict, the formation of pharmaceutical solids with tailored properties.

We are delighted to edit this Special Issue called "Multicomponent Pharmaceutical Solids". New multicomponent pharmaceutical materials have come to stay, and we will soon see relevant applications on multidrug resistance or co-drug synergy thanks to their rational design. Until then, this Special Issue is devoted to compiling all current aspects related to the field of multicomponent pharmaceutical solids. We hope that this volume is of interest to all *Crystals* readers.

> **Duane Choquesillo-Lazarte and Alicia Dom´ınguez Mart´ın** *Editors*
