During the period October 1969 to April 1972 a comprehensive study of the reproductive biology of the Norway lobster, Nephrops norvegicus (L. 1758), was made in the Irish Sea. Where possible comparison has been made with data from other areas. The results of the present investigation show that most females are berried annually, egg-laying taking place in August and September, and hatching in April to June of the following year. Moulting and copulation occur in mature females during May to August. In contrast males do not show a marked reproductive cycle, spermatogenesis occurring throughout the year. The method of fertilization is discussed, and it is suggested that this may be internal. Copulatory and egg-hatching behaviour is described, together with details of egg-laying and incubation in the laboratory. A pre-zoeal larval stage is described for the first time, bringing the total number of larval instars to four. The terminology of the larval stages is revised.