In contrast to many investigations of cellular automata with regard to their ability to accept inputs under certain time constraints, we are studying here cellular automata towards their ability to generate strings in real time. Structural properties such as speed-up results and closure properties are investigated. On the one hand, constructions for the closure under intersection, reversal, and length-preserving homomorphism are presented, whereas on the other hand the non-closure under union, complementation, and arbitrary homomorphism is obtained. Finally, decidability questions such as emptiness, finiteness, equivalence, inclusion, regularity, and context-freeness are addressed.