Aluminum capacitors have relatively high equivalent series resistance (ESR), which means that they generate high heat for a given current level (solid aluminum variants improve this behavior). Aluminum capacitors are low cost, have a high capacitance value and high energy density value. Energy density determines the amount of energy that may be stored in a given volume.

Aluminum capacitors implement basic three-layer capacitor structure conductor–dielectric–conductor in the form of the following: aluminum–aluminum oxide–electrolyte. Because they utilize electrolytes, aluminum capacitors are a subgroup of the electrolytic capacitors. They are polarized so they must be connected to positive voltage only. Application of negative voltage leads to the failure of the capacitor.

According to the type of electrolyte used, aluminum capacitors are further subdivided into two groups: non-solid (wet) aluminum capacitors (which use liquid electrolytes) and solid aluminum capacitors (which use solid electrolytes).

Liquid electrolyte dries in time. Drying leads to capacitor failure and limits its lifetime. Non-solid capacitors are sealed to suppress drying and leakage of electrolyte. Sealing introduces an additional problem. The occurrence of irregular working condition leads to the explosion of the capacitor because excessive pressure is built up inside. For example, application of negative voltage or excessive current stimulates chemical reactions and/or overheating within the capacitor, which produces evaporation and expansion of electrolyte in a sealed capacitor. In order to prevent burst and explosion during failure, liquid electrolytic capacitors are manufactured with pressure relief vents.

Low price significantly contributes to the widespread usage of this capacitor type. They can be found in diverse types of electrical and electronic equipment.