Flour

The most common flour used in baking is wheat-based, which contains proteins that, when agitated in the presence of water, form gluten. Gluten is a stretchy protein structure that holds in the bubbles made by leaveners (such as yeast or baking powder). Flours are graded by the amount of protein — in the United States, high gluten is 12-14%, while the standard bread flour contains 10-13% protein. All-purpose flour has a protein content of 9-12%, and pastry flour, used primarily for baking, is a softer flour with a protein content of about 9-10%. In Europe, the flour generally contains a lower protein content than that found in American bread flour, which necessitates longer mixing times and additions to the flour, such as ascorbic acid (a form of vitamin C, which gives the dough tenacity for better rising).

Water

Many bakers prefer to use filtered water. Tap water that contains any traces of chlorine should not be used because the chlorine can kill the yeast.

Yeast

Two basic kinds of yeast are used in bread making: commercial yeast and natural yeast, which is the kind of yeast contained in sourdoughs. The two most common types of commercial yeast are compressed, also called cake yeast, and dry yeasts of which there are several forms. Active dry yeast must first be “proofed” to activate it prior to adding it to the flour and water: it is soaked in a small amount of water between 90-110F or 32-43C for approximately ten minutes. Sugar can be added to help the yeast “feed” and proof faster. Instant dry yeast may be mixed into flour before adding water.

Natural yeast is trickier to work with, and it must be kept in controlled environments. By combining flour and water and allowing it to sit, the mixture will pick up natural yeasts in the air that will ferment to create more yeast. With repeated stirring and additions of flour and water, the mixture will become a sourdough. The sourness of the dough comes from a bacterial culture that lives alongside the natural yeast, typically a lactobacilli strain, most famously "Lactobacillus sanfranciscensis", as in the famous San Francisco sourdough.

Professional dough conditioners are available to enhance the environment for the yeast.

Salt

Salt serves a more important function than just providing flavor; it also controls the fermentation of the yeast. Too much salt and the yeast action will be retarded; too little salt, and the dough will not entrap enough carbonic gases to form a strong texture. Certain breads, such as Italian ciabattas, do not traditionally contain salt, but the majority of breads do.