>So you have two strings - one with say 3,000 words and another of about 20K with x words. If the likelihood of words in the 'search for' string not appearing in the 'search in' string is fairly high you could use linq to obtain a list of words that *did* occur and use that as a basis for further manipulation. Something like:

static public List<string> WordsInTarget(string searchFor, string searchIn)
>        {
>            List<string> s = (from x in searchIn.Split() select x).ToList();
>            return (from x in searchFor.Split().Where(x => s.Contains(x)) select x).Distinct().ToList();
>        }

No idea how it would compare speed-wise tho. And Split() might need some parameters....

The split is already implemented. The optimization will probably move up one layer when using SubString().