Composite electrolytes for applications in batteries and supercapacitors, i.e., in electrochemical energy technology, are gaining growing attention. In the absence of a commonly accepted definition, binary and ternary combinations of materials, e.g., a polymer with an electrolyte salt or electrolyte salt solution and a third conductivity- and performance-enhancing constituent, are assumed as a definition of a composite electrolyte in the following review. Relevant fundamentals and reported research results, including explanations of the described effects of added ingredients and achieved improvements, are reviewed. Future perspectives and directions of further research are sketched.