Improving production of shale gas and tight oil from low-permeability formations has dominated the energy landscape over the past two decades. Successful applications of horizontal drilling combined with hydraulic fracturing (HF) techniques, first demonstrated in the Texas Barnett shales in the 1990s, are the impetus for the current shale boom in the Americas. Chemicals are at the heart of the fracturing (or "fracking") process, which includes pumping a water-based fluid and proppant into a shale formation to create fissures that access the buried hydrocarbons. The fracturing fluid composition is critical to the completion design and should reduce wellbore friction while enabling proppant placement into the fractures. However, adding too many chemicals without properly evaluating the fracturing fluid formulation can lead to increased cost, poor fluid performance, or even damage to the reservoir. This review discusses the purpose of each chemical used in HF fluids, the techniques applied by the production chemist to assess fluid performance, and suggested best practices in optimizing formulations to reduce costs, enhance performance, and improve footprint.