Linked from
The 10 pages that link to Morse theory, each with the reason it gives.
Level setRelated: Changes in level-set topology occur as the value crosses critical values.
Saddle pointRelated: The number of descending directions at a saddle contributes to its Morse index.
Sard's theoremRelated: Sard's theorem supplies regular levels between critical values.
Cell (topology)Related: Morse theory often builds a space by adding one cell at each critical point.