The goal of this book is to address a range of topics that affect the recovery of hydrocarbons from extremely low-permeability unconventional oil and gas reservoirs. While there are various definitions of unconventional reservoirs, in this book we consider oil- and gas-bearing formations with permeabilities so low that economically meaningful production can only be realized through horizontal drilling and multi-stage hydraulic fracturing. These reservoirs have permeabilities measured in nanodarcies, not millidarcies – in other words, a million times lower than conventional reservoirs. Despite their ultra-low permeability, there is no question about the scale and impact of production from unconventional oil and reservoirs in the US and Canada over the past decade.