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: iShares Core S&P 500 UCITS ETF (Acc) · as of 2026-07-10 · 504 holdings

#	Stock	Weight
1	NVIDIA CORP NVDA	7.84 %
2	APPLE INC AAPL	7.10 %
3	MICROSOFT CORP MSFT	4.39 %
4	AMAZON.COM INC AMZN	3.68 %
5	ALPHABET INC CLASS A GOOGL	3.21 %
6	BROADCOM INC AVGO	2.90 %
7	ALPHABET INC CLASS C GOOG	2.57 %
8	META PLATFORMS INC CLASS A META	2.25 %
9	TESLA INC TSLA	1.76 %
10	MICRON TECHNOLOGY INC MU	1.69 %

Sectors

IT37.6 %

Finanzwesen12.1 %

Kommunikation9.9 %

Zyklische Konsumgüter9.2 %

Gesundheitsversorgung8.9 %

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.