A cell expansion system comprising: a) a first fluid circulation path comprising i) a first fluid flow path having at least opposing ends (18, 20), a first opposing end of said first fluid flow path fluidly associated with a first inlet port of a hollow fiber membrane-containing cell growth chamber and a second opposing end of said first fluid flow path fluidly associated with a first outlet port of said cell growth chamber, wherein said first fluid flow path is fluidly associated with the intracapillary portion of said hollow fiber membrane, and ii) a first fluid controller operably associated with said first fluid flow path; b) a second fluid circulation path comprising i) a second fluid flow path having at least opposing ends, a first opposing end of said second fluid flow path fluidly associated with a second inlet port of said cell growth chamber and a second opposing end of the second fluid flow path fluidly associated with an outlet port of said cell growth chamber, wherein the second fluid flow path is fluidly associated with the extracapillary portion of said hollow fiber membrane; and ii) a second fluid controller operably associated with said second fluid flow path; c) a first fluid inlet path fluidly associated with said first circulation path to allow fluid into said first circulation path; d) a first fluid outlet path fluidly associated with at least one of said first or second circulation paths; e) at least one fluid connector path having at least opposing ends, a first opposing end of said fluid connector path fluidly associated with said first fluid circulation path and a second opposing end of said fluid connector path fluidly associated with said second fluid circulation path; and f) at least a second fluid connector flow path having opposing ends, one end of said second fluid connector flow path fluidly associated with said first fluid flow path and a second end of said second fluid connector flow path fluidly associated with said second fluid flow path. a) a first fluid circulation path comprising i) a first fluid flow path having at least opposing ends (18, 20), a first opposing end of said first fluid flow path fluidly associated with a first inlet port of a hollow fiber membrane-containing cell growth chamber and a second opposing end of said first fluid flow path fluidly associated with a first outlet port of said cell growth chamber, wherein said first fluid flow path is fluidly associated with the intracapillary portion of said hollow fiber membrane, and ii) a first fluid controller operably associated with said first fluid flow path; i) a first fluid flow path having at least opposing ends (18, 20), a first opposing end of said first fluid flow path fluidly associated with a first inlet port of a hollow fiber membrane-containing cell growth chamber and a second opposing end of said first fluid flow path fluidly associated with a first outlet port of said cell growth chamber, wherein said first fluid flow path is fluidly associated with the intracapillary portion of said hollow fiber membrane, and ii) a first fluid controller operably associated with said first fluid flow path; b) a second fluid circulation path comprising i) a second fluid flow path having at least opposing ends, a first opposing end of said second fluid flow path fluidly associated with a second inlet port of said cell growth chamber and a second opposing end of the second fluid flow path fluidly associated with an outlet port of said cell growth chamber, wherein the second fluid flow path is fluidly associated with the extracapillary portion of said hollow fiber membrane; and ii) a second fluid controller operably associated with said second fluid flow path; i) a second fluid flow path having at least opposing ends, a first opposing end of said second fluid flow path fluidly associated with a second inlet port of said cell growth chamber and a second opposing end of the second fluid flow path fluidly associated with an outlet port of said cell growth chamber, wherein the second fluid flow path is fluidly associated with the extracapillary portion of said hollow fiber membrane; and ii) a second fluid controller operably associated with said second fluid flow path; c) a first fluid inlet path fluidly associated with said first circulation path to allow fluid into said first circulation path; d) a first fluid outlet path fluidly associated with at least one of said first or second circulation paths; e) at least one fluid connector path having at least opposing ends, a first opposing end of said fluid connector path fluidly associated with said first fluid circulation path and a second opposing end of said fluid connector path fluidly associated with said second fluid circulation path; and f) at least a second fluid connector flow path having opposing ends, one end of said second fluid connector flow path fluidly associated with said first fluid flow path and a second end of said second fluid connector flow path fluidly associated with said second fluid flow path.