The mash tun is where the grist, or grain, mixes with the water. Additional adjuncts can be added at this stage as well. The mash tun is partially filled with hot water prior to the grist being introduced. The water can be mixed prior to entry by means of a Steel's Masher, or it can be sprayed upon entry using a sparge arm. The Steel's Masher forces the combination of water and grist through rotating plates prior to entry of the mash tun. If a Steel's Masher is not in place it simply means that there will be more agitation in the bottom of the mash tun.

The temperature of the water is heated to a higher temperature than the anticipated mashing temperature. This is to compensate for the temperature of the grain. This can be easily calculated based on the weight of the grist and amount of water used. In practice the the brew kettle or boiler is generally used for heating the water. In some instances an additional steamer is used. Temperature control systems are used to maintain the desired temperature, and steam injection offers a very high degree of precision.

Large mash tuns have a speed controlled motor driven agitation arm. This is used to properly mix the ingredients for consistency and to also make sure that larger fragments of the grains are penetrated. The mash tun is insulated to maintain temperature and this is done using foam or sometimes cork. Stainless steel and copper are the most common materials for inside the vessel. A man hole is also part of the mash tun and allows for the removal of spent grains.

Decoction mashing is achieved by rising the temperature of the mash in steps. This is achieved by withdrawing a portion of wort from the mash tun, and raising it to a higher temperature. This is then added back to the mash tun, raising the overall temperature of the wort.

In some cases the lauter tun and mash tun are combined into a single piece of equipment. This reduces overall expense of equipment and reduces floor space requirements. The essential requirements for a lauter tun are a sparging arm, and a false bottom for filtering the grains. Also it is common to see a boiler and mash tun combined into a single vessel, with the boiler on bottom.

Mash tuns come in varying sizes from 1 BBL to 50 BBL, and the largest breweries go as high as 500 BBL. They are measured in both HL and BBL. A typical brewpub may use a 5,7, or 15 BBL sized mash tun.