Languages appeared from the very beginning in membrane computing, by their length sets or directly as sets of strings. We briefy recall here this relationship, with some details about certain recent developments. In particular, we discuss the possibility to associate a control word with a computation in a P system. An improvement of a result concerning the control words of spiking neural P systems is given: regular languages can be obtained as control words of such systems with only four neurons (and with usual extended rules: no more spikes are produced than consumed). Several research topics are pointed out.