The breeding biology of Marsh Warblers is described, based on studies in 1978 in the SW-Veluwe (c. 100 ha) and in 1983 in ESE-Drenthe (11,3 ha). A discrepancy was found between the number of teritorial males (by means of the mapping method) and the number of active nests (table 1). It was thought that all nests were found. A detailled description is given of the methods of nestfinding. Most Marsh Warblers in ESE-Drenthe started nestbuilding one week after the arrival in the breeding habitats, sometimes even later (figure 1). This delay (cf. Dowsett-Lemaire 1981) was probably largely caused by the prolonged unfavourable weather during May 1983. Egg-laying took place from the last decade of May up to the end of June, with one laying peak in the first decade of June. The tail in the figure of egg-laying was mainly composed of repeat-layings (figure 2). On the SW-Veluwe 44 pairs started egg-laying, resulting in 43 completed clutches: 5x C/3, 17x C/4, 19x C/5 and 2x C/6 (mean 4.4). In ESE-Drenthe 27 pairs produced 1x C/3, 15x C/4, 10x C/5 and 1x C/6 (mean 4.4). Clutch size decreased with progressing breeding season (figure 3). Fourty-three pairs on the SW-Veluwe produced 190 eggs, resulted in 142 hatched eggs and 126 fledglings (2.9 young/pair or 4.3 young/succesfull pair). In ESE-Drenthe 27 pairs laid 119 eggs, of wich 72 hatched and 52 fledged (1.6 young/pair or 3.5 young/succesful pair)(table 2). Causes of failure were predation, disturbances with desertion as a result and mowing of the breeding habitat, resulting in the destruction of nests (table 3).