Indexing

How SQL Server Indexes Work

Index Overview - the basics of how indexes work.
Data Pages & Extents - about how indexes and data are stored on disk.
Table and Record Structure - more about the mechanics of data storage.
Effects of Data Modifications - the more indexes you have, the longer inserts/updates/delete take.
Index Statistics - the basics of how statistics affect query execution.

Included Columns - starting with SQL Server 2005, you can add fields to indexes with a smaller performance hit.
Indexed Views - how to build indexes on top of views for more speed.
XML Indexes Overview - how to index XML documents and fragments.
XML Index Rules - some additional considerations and requirements for XML indexes.

Indexing Strategies - choosing between clustered/nonclustered, long/short, selectivity, and more.
Best Practices for Creating Indexes - tips to start with when designing a new index strategy.
Choosing Index Keys - how to pick which fields to use for a primary key.
Index Selectivity and Column Order - how to pick which columns to put first, and how it affects different kinds of queries.
Wide Index Performance - does adding more fields to an index cause it to run slower?
Tuning Indexes - including articles on the Index Tuning Wizard and Database Tuning Advisor.

Indexes need regular maintenance in order to perform well.

Index Maintenance Tutorial - video and script explaining how to defragment and rebuild your indexes automatically.
Index Maintenance in SQL Server 2005 and Beyond - SQL 2005 introduced new syntax to ALTER INDEX.
Index Maintenance Overview - analyzing fragmentation and statistics to get the best performance possible out of your indexes.