This dissertation investigates to what extent parliaments have a legislative function in authoritarian regimes. Arguably bolstering the legitimacy of authoritarian systems, power sharing institutions give politically relevant actors access to policy making. This analysis distinguishes two forms of power sharing (‘contestation’ and ‘differentiation’) and examines their impact on the legislative activity of parliaments. Legislative activity is measured in terms of size and scope, and refers to parliamentary initiative, as well as the amendment and delay of executive bills. Contestation, defined as multiparty elections, supposedly stimulates legislative activity by giving legislators a stronger bargaining position vis-à-vis the leadership. Higher levels of contestedness enhance the credibility of defection and require legislative concessions to potential defectors to secure regime stability. Alternatively, differentiation, referring to the dispersion of decision-making responsibilities in a political regime, affects legislative activity by marking the parliament’s proximity to the government and through the delegation of legislative responsibilities to institutional positions in the parliament. The empirical section consists of statistical analyses and paired comparisons of similar cases, focusing on legislatures in Russia, Kazakhstan, Ukraine and Kyrgyzstan that vary in terms of contestedness and differentiation. The analysis is based on two original datasets with information on 12,712 enacted laws and 6,693 MPs in the selected countries between 1998 and 2016. The results show that the selected legislatures, on average, initiated 38% of all enacted laws, rewrote executive bills, on average, for 43% and spent months deliberating on each executive bill. The results show that power sharing arrangements provide important explanations for this activity. Members of parliament increase their legislative activity when the level of contestedness is high and when the parliament’s constitutional leverage vis-à-vis the government is strong. The most important legislators are, however, allies of the ruler who have a position with institutionally delegated legislative power.