The interest in arbitrary spreads arose in the fifties and sixties of the 20th century in connection with the problem to construct certain geometric structures, namely so-called translation planes. In this context the elliptic congruence of lines is also called a regular spread.
The essential problem is to find spreads that are non-regular. It is very easy to find such spreads by modifying a regular spread. However, in general simple constructions yield non-continuous spreads. It is still not too difficult to find non-regular continuous spreads, but it is highly non-trivial to find smooth spreads other than the regular ones. In particular, just recently the first examples of non-regular algebraic spreads have been discovered. In this lecture we will illustrate some of these results by giving explicit examples.