We study non-orientable Seifert surfaces for knots in the 3-sphere, 
and examine their boundary slopes. 
In particular, it is shown that for a crosscap number two knots, 
there are at most two slopes which can be the boundary slope of its minimal 
genus non-orientable Seifert surface, and an infinite family of knots with two 
such slopes will be described. Also, we discuss the existence of 
essential non-orientable Seifert surfaces for knots.