Large　reciprocating　compressors　are　important　equipment　used　in　a　wide　range　of　process　industries.　Most　of　these　compressors　have　huge　power　requirements　and　their　capacity　often　needs　to　be　regulated.　Advanced　technologies　in　compressor　capacity　control　are　effective　approaches　for　saving　large　amounts　of　energy　in　process　industries.　This　paper　reviews　the　basic　theories　and　the　state　of　the　art　of　the　capacity　control　technologies.　In　particular,　a　compressor　working　procedure　is　first　presented　using　an　analogy　to　the　pulse　signal　concept.　Compressor　capacity　control　methods　are　classified　into　pulse　frequency　modulation　and　pulse　amplitude　modulation　from　a　perspective　of　pulse　signal　processing.　The　mechanisms　and　feasibility　of　some　important　methods,　including　dead　volume　variable　control,　partial-stroke　and　full-stroke　suction　valve　opening,　are　reviewed.　Based　on　the　pulse　signal　concept,　a　duty　cycle　regulation　method　for　capacity　control　is　introduced,　and　the　performance　and　implementation　of　the　new　method　are　compared　with　those　of　the　existing　suction　valve　opening　methods.　The　duty　cycle　regulation　method　has　integrated　advantages　over　the　other　methods　in　terms　of　regulating　precision,　pressure　stability,　energy　saving　and　reliability.　All　the　suction　valve　opening　methods　can　cause　gas　reflux,　resulting　in　the　so-called　breathing　effect.　The　breathing　effect　has　negative　effects　on　regulation　performance　and　compressor　security,　which　needs　further　investigation　in　the　future.