What is an index?

An index is a rules-based list of securities that measures a market – like the S&P 500 (500 large US firms) or the MSCI World (~1,400 firms across developed markets). An ETF simply copies an index.

Indices are usually market-cap weighted: bigger companies get a bigger share. Here’s a live example:

Live data

What's inside – S&P 500

Based on: State Street® SPDR® S&P® 500 UCITS ETF (Acc) · as of 2026-08-26 · 504 holdings

#	Stock	Weight
1	NVIDIA Corporation NVDA	7.69 %
2	Apple Inc. AAPL	6.97 %
3	Microsoft Corporation MSFT	5.58 %
4	Amazon.com Inc. AMZN	3.86 %
5	Alphabet Inc. Class A GOOGL	3.04 %
6	Broadcom Inc. AVGO	2.55 %
7	Alphabet Inc. Class C GOOG	2.43 %
8	META Platforms Inc. Class A META	1.92 %
9	Micron Technology Inc. MU	1.60 %
10	Tesla Inc. TSLA	1.47 %

Sectors

IT37.0 %

Finanzwesen12.4 %

Kommunikation9.6 %

Gesundheitsversorgung9.5 %

Zyklische Konsumgüter9.2 %

Industrie8.6 %

The weights above aren’t chosen by a manager – they follow the index rules and update as prices move.

Why it matters

The index decides what you own and how concentrated it is.
Two ETFs on the same index are nearly identical inside – then pick by cost .
Browse all indices we cover in the indices overview .

Information only, not investment advice.