This study examines the extent to which Muslim self-identification and mosque attendance have changed in the period 1997–2009 among people of Turkish and Moroccan descent in the Netherlands. Mainly trendless fluctuations are found. Overall, Muslim self-identification seems to very slightly increase and mosque attendance seems to very slightly decrease. We examined the extent to which factors that are important according to theories and previous research explain or enlarge these differences over time. The factors about which we hypothesize are largely unable to explain differences over time in Muslim self-identification and mosque attendance.