A hydrogen generator (10; 210; 310; 410; 510) comprising: a cartridge (20; 220; 320; 420; 520), the cartridge comprising: a casing (22, 24, 26; 222; 422; 522); a plurality of pellets (40; 240; 340; 440; 540), each comprising at least one hydrogen containing material capable of releasing hydrogen gas when heated; a plurality of solid heat transfer members (44; 244; 344; 444; 544), each in direct contact with but not penetrating the casing (22, 24, 26; 222; 422; 522); a hydrogen outlet valve in the casing (22, 24, 26; 222; 422; 522); and a hydrogen flow path from each fuel pellet to the hydrogen outlet valve; a compartment (12; 212; 512) configured to removably contain the cartridge (20; 220; 320; 420; 520), the compartment (12; 212; 512) comprising: a housing comprising a wall (14; 514); a hydrogen outlet through the housing; a cavity within the housing within which the cartridge (20; 220; 320; 420; 520) can be disposed; and plurality of heating elements (50; 250; 550) disposed within the housing such that when the cartridge (20; 220; 320; 420; 520) is disposed within the cavity each heating element (50; 250; 550) is in contact with an outer surface of the cartridge casing and disposed so that heat can be conducted from the heating element (50; 250; 550), through the casing and to a heat transfer member (44; 244; 344; 444; 544), which can conduct the heat to a portion of the at least one hydrogen containing material not in contact with the casing, wherein each pellet (40; 240; 340; 440; 540) is in contact with a heat transfer member (44; 244; 344; 444; 544); and an initiation system comprising the heat transfer members (44; 244; 344; 444; 544), the heating elements (50; 250; 550), and circuitry for conducting an electric current to the heating elements (50; 250; 550); wherein the electrical current can be applied selectively to one or more heating elements (50; 250; 550) for generating heat to selectively heat one or more pellets (40; 240; 340; 440; 540) to initiate a release of hydrogen gas. a cartridge (20; 220; 320; 420; 520), the cartridge comprising: a casing (22, 24, 26; 222; 422; 522); a plurality of pellets (40; 240; 340; 440; 540), each comprising at least one hydrogen containing material capable of releasing hydrogen gas when heated; a plurality of solid heat transfer members (44; 244; 344; 444; 544), each in direct contact with but not penetrating the casing (22, 24, 26; 222; 422; 522); a hydrogen outlet valve in the casing (22, 24, 26; 222; 422; 522); and a hydrogen flow path from each fuel pellet to the hydrogen outlet valve; a casing (22, 24, 26; 222; 422; 522); a plurality of pellets (40; 240; 340; 440; 540), each comprising at least one hydrogen containing material capable of releasing hydrogen gas when heated; a plurality of solid heat transfer members (44; 244; 344; 444; 544), each in direct contact with but not penetrating the casing (22, 24, 26; 222; 422; 522); a hydrogen outlet valve in the casing (22, 24, 26; 222; 422; 522); and a hydrogen flow path from each fuel pellet to the hydrogen outlet valve; a compartment (12; 212; 512) configured to removably contain the cartridge (20; 220; 320; 420; 520), the compartment (12; 212; 512) comprising: a housing comprising a wall (14; 514); a hydrogen outlet through the housing; a cavity within the housing within which the cartridge (20; 220; 320; 420; 520) can be disposed; and plurality of heating elements (50; 250; 550) disposed within the housing such that when the cartridge (20; 220; 320; 420; 520) is disposed within the cavity each heating element (50; 250; 550) is in contact with an outer surface of the cartridge casing and disposed so that heat can be conducted from the heating element (50; 250; 550), through the casing and to a heat transfer member (44; 244; 344; 444; 544), which can conduct the heat to a portion of the at least one hydrogen containing material not in contact with the casing, wherein each pellet (40; 240; 340; 440; 540) is in contact with a heat transfer member (44; 244; 344; 444; 544); and a housing comprising a wall (14; 514); a hydrogen outlet through the housing; a cavity within the housing within which the cartridge (20; 220; 320; 420; 520) can be disposed; and plurality of heating elements (50; 250; 550) disposed within the housing such that when the cartridge (20; 220; 320; 420; 520) is disposed within the cavity each heating element (50; 250; 550) is in contact with an outer surface of the cartridge casing and disposed so that heat can be conducted from the heating element (50; 250; 550), through the casing and to a heat transfer member (44; 244; 344; 444; 544), which can conduct the heat to a portion of the at least one hydrogen containing material not in contact with the casing, wherein each pellet (40; 240; 340; 440; 540) is in contact with a heat transfer member (44; 244; 344; 444; 544); and an initiation system comprising the heat transfer members (44; 244; 344; 444; 544), the heating elements (50; 250; 550), and circuitry for conducting an electric current to the heating elements (50; 250; 550); wherein the electrical current can be applied selectively to one or more heating elements (50; 250; 550) for generating heat to selectively heat one or more pellets (40; 240; 340; 440; 540) to initiate a release of hydrogen gas.