In federal and regionalised states, bicameralism constitutes shared rule between levels of governments. At the same time, second chambers serve as a safeguard protecting selfrule of decentralised governments against the encroachments of central legislation into their areas of responsibility. Both functions seem to be best fulfilled in legislative systems requiring joint decisions of legislative chambers. Depending on particular conditions, joint decision-making involves the risk that legislation ends with ineffective compromises or even fails. Under favourable conditions, it provides a productive structure to apply shared rule and protect self-rule. Comparative studies can identify these conditions, and appropriate ways to adjust institutional designs of bicameralism accordingly, bearing in mind that significant institutional reforms of bicameral systems are difficult to achieve.