An enhanced vapor injection scroll compressor, comprising: a compressor housing; a main frame disposed in the compressor housing; an orbiting scroll disposed on the main frame, wherein the orbiting scroll comprises: an orbiting scroll end plate having a side end surface adjacent the main frame and a side end surface distanced from the main frame; an orbiting scroll wrap disposed on the side end surface distanced from the main frame, and wherein a back pressure chamber is defined between the orbiting scroll end plate and the main frame; a fixed scroll disposed on the orbiting scroll, wherein the fixed scroll comprises: a fixed scroll end plate having a side end surface adjacent the orbiting scroll and a side end surface distanced from the orbiting scroll; a fixed scroll wrap disposed on the side end surface adjacent the orbiting scroll, and wherein the fixed scroll wrap and the orbiting scroll wrap operatively mesh each other to form a crescent-shaped compression cavity; and characterized by: a first medium pressure passage and a second medium pressure passage; wherein the first medium pressure passage is defined in the orbiting scroll, and the second medium pressure passage is defined in the fixed scroll; and wherein during rotation of the orbiting scroll, at least one of the first medium pressure passage and the second medium pressure passage is configured to connect the crescent-shaped compression cavity with the back pressure chamber. a compressor housing; a main frame disposed in the compressor housing; an orbiting scroll disposed on the main frame, wherein the orbiting scroll comprises: an orbiting scroll end plate having a side end surface adjacent the main frame and a side end surface distanced from the main frame; an orbiting scroll wrap disposed on the side end surface distanced from the main frame, and wherein a back pressure chamber is defined between the orbiting scroll end plate and the main frame; an orbiting scroll end plate having a side end surface adjacent the main frame and a side end surface distanced from the main frame; an orbiting scroll wrap disposed on the side end surface distanced from the main frame, and wherein a back pressure chamber is defined between the orbiting scroll end plate and the main frame; a fixed scroll disposed on the orbiting scroll, wherein the fixed scroll comprises: a fixed scroll end plate having a side end surface adjacent the orbiting scroll and a side end surface distanced from the orbiting scroll; a fixed scroll wrap disposed on the side end surface adjacent the orbiting scroll, and wherein the fixed scroll wrap and the orbiting scroll wrap operatively mesh each other to form a crescent-shaped compression cavity; and characterized by: a first medium pressure passage and a second medium pressure passage; wherein the first medium pressure passage is defined in the orbiting scroll, and the second medium pressure passage is defined in the fixed scroll; and wherein during rotation of the orbiting scroll, at least one of the first medium pressure passage and the second medium pressure passage is configured to connect the crescent-shaped compression cavity with the back pressure chamber. a fixed scroll end plate having a side end surface adjacent the orbiting scroll and a side end surface distanced from the orbiting scroll; a fixed scroll wrap disposed on the side end surface adjacent the orbiting scroll, and wherein the fixed scroll wrap and the orbiting scroll wrap operatively mesh each other to form a crescent-shaped compression cavity; and characterized by: a first medium pressure passage and a second medium pressure passage; wherein the first medium pressure passage is defined in the orbiting scroll, and the second medium pressure passage is defined in the fixed scroll; and wherein during rotation of the orbiting scroll, at least one of the first medium pressure passage and the second medium pressure passage is configured to connect the crescent-shaped compression cavity with the back pressure chamber. a first medium pressure passage and a second medium pressure passage; wherein the first medium pressure passage is defined in the orbiting scroll, and the second medium pressure passage is defined in the fixed scroll; and wherein during rotation of the orbiting scroll, at least one of the first medium pressure passage and the second medium pressure passage is configured to connect the crescent-shaped compression cavity with the back pressure chamber.