The most important effect of roasting beans is that the flavor is developed through the complex chemical changes caused by heat, which is the process of pyrolysis. It is estimated that a coffee bean contains more than 2,000 chemical substances, which may be broken down or changed during roasting into hundreds of "volatile aroma compounds". Various acids, oils, proteins, vitamins, sugars, starches, and caffeine are altered; some are enhanced and some are diminished. In certain cases, some substances are both developed and then burned away if the roasting time is extended.

A light roast is seldom used commercially, as it shows up all the flaws inherent in beans, many of which will disappear, or at least be hidden by other flavors, in a dark roast. For example, if a coffee has the distinctive, sometimes unusual, pleasant and expensively-acidic qualities of a high-grown Arabica, it is better for it not to be dark-roasted. The darker the roast, the more uniform all coffees taste, as a truly dark roast will overwhelm the taste-buds, allowing them to perceive nothing of the coffee itself. A darker roast may sweeten some coffees, but only to a point; past a certain degree of roast all coffee becomes bitter. Also, the darker the roast, the greater the loss of acidity, that most sought-after quality.

Coffee roasters all vary in size and capacity, but the roasting process changes little from one size to another. All equipment should be pre-heated to a roasting temperature some minutes before the green beans are added, so all surfaces are uniformly hot. Many roasters are equipped with a revolving drum, often lined internally with curved metal strips which constantly toss the beans towards the centre of the drum. Above all, beans must be kept moving if they are to roast evenly, without burning. In fact, if a drum stops revolving while the heat is still on and the beans are hot, there is a danger of instant combustion within the drum. (The saying goes, "You're not a coffee roaster 'til you've had your first fire!")