This chapter addresses a fascinating and key aspect of human learning-informal learning in online communities of practice. Informal learning refers to experiential learning, that is, learning outside of the classroom. Sharing has enabled humans to survive and to thrive, and, as discussed in this book, has been the basis of civilizational advances. Appreciating the collective nature of knowledge is especially important in an age when almost every field changes too much, too fast for individuals to master. What makes managing knowledge a challenge is that it is not an object that can be stored, owned, and moved around like a piece of equipment or a document. The concept of community is a difficult term to define because of the very wide and diffuse use of the term. The term "community of practice" has evolved from an emphasis on apprenticeship within an organization to that of members sharing a common profession or type of work beyond an institutional affiliation.