I took the excuse of a keynote I was asked to give at Slush's "On-Demand Everything" conference to put a few slides together, which summarize my thoughts on the category, which I've defined as mobile-first marketplaces connecting consumers to professionals offering a service (ie. not a product), in less than an hour. These companies represent the 4th generation of online marketplaces, which evolved over time from simple, generalist, passive classifieds (Craigslist), where all the work of matching supply and demand was left to the buyers and sellers themselves, to ever more specialised and intelligent platforms, able to offer almost perfect and instantaneous matching, requiring virtually no work from both sides of the transaction (Uber's "click of a button" experience). While it's safe to assume that every service will move towards such a seamless mobile experience, the pace at which the change is occuring, and whether it will be led by a new startup vs. an incumbent, depends on a few characteristics, which vary from sector to sector. The following questions that will determine the pace of adoption and the eventual success of on-demand startups: 1. How urgent is the need? 2. How frequent is the need? 3. How complex is the task? 4. Is mobile a game-changer? 5. Can it ever be profitable?