It depends on the length of the preceding vowel: "ß" is only used after a long vowel (or diphthong). The "i" in "wissen" is short, so it has to be "ss" there. See some more details, particularly on whether it's one of the two or simply "s", in Wikipedia: http://de.wikipedia.org/wiki/ß#Heutige_Rechtschreibregeln http://en.wikipedia.org/wiki/ß#Current_usage_in_German

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Danke schön, jetzt ich weiß.

Yes, ...DonHolgos answer is right. To decide if you use ss or ß you have to watch to the vowel before ss or ß. Example: "Straße" (street) is pronounced with a long a, like Straaaaaße. So you choose ß in the word Straße. "vermissen" (to miss) has a short i, so you use ss in the word vermissen.