We are pleased to introduce to our readers a special collection of BioEssays on phosphoinositides (PtdInsPs), which can be found at: http://onlinelibrary. wiley.com/journal/10.1002/%28ISSN% 291521-1878/homepage/phosphoinositides_special_collection.htm. PtdInsPs are versatile phospholipid second messengers that play an important role in many aspects of cell function including cell proliferation, signal transduction, organelle biogenesis and identity, membrane trafficking, and even nuclear dynamics. This diverse functional portfolio arises in part because (i) each PtdInsP has a specialized sub-cellular localization, (ii) each PtdInsP species regulates its own set of protein effectors, which can number in the dozens, (iii) PtdInsPs can be interconverted by a series of lipid kinases and phosphatases, and (iv) PtdInsP signaling is integrated and cross-talks with other molecular modulators such as members of the Rab family of small GTPases. Thus, despite being “simple” lipids, understanding the regulation, function, and relationship between each PtdInsP poses a tremendous challenge. Nevertheless, given their significance in cell physiology, health, and disease, it is imperative that we address paucities in our understanding of PtdInsP function and regulation. To help achieve this, we hope that this special issue of BioEssays will be a valuable resource for specialists and non-specialists to review and learn about each PtdInsP and the numerous fundamental questions that remain unanswered. The special collection is composed of seven articles, one for each of the seven PtdInsP species. Importantly, we plan to append additional relevant articles over time. Starting with the mono-phosphorylated PtdInsP species,