The paper discusses polymer mixed ionic electronic conductors (p-MTECs) and the I-V relations of Galvanic cells based on p-MIECs with ion blocking electrodes. As a preparation for the discussion of p-MIECs, we first review shortly conjugated polymer semiconductors and polymer electrolytes. It is shown that the conjugated polymer semiconductors are MIECs, not purely electronic conductors. On the other hand, electronic conduction can be neglected in the polymer electrolytes. The best p-MTECs are blends of polymer electrolytes and conjugated polymer semiconductors.