As a change in current investment strategies to passive investments is discernible, there is a need to highlight the underlying instruments of investments products as they complete the full range of investment products like passive- and exchange traded funds, certificates and derivatives. Whereas the modern literature aims highly on covering index usability with regards to derivatives, the development of principle concepts concerning index construction over time as well as their impact on the financial industry, besides increasing volumes held by products replicating indices, are scarcer highlighted. Their increasing importance with regards to referred capital in financial products is incontestable and the explicit regulations carried out the last years concerning benchmarks illustrate the common sense that indices have reached the broad private investor base through various investment products.