Complex behavior change interventions need evidence regarding the effectiveness of individual components to understand how these interventions work. The objective of this study was to identify the least and most promising behavior change techniques (BCTs) within the Dutch intervention "A Matter of Balance" (AMB-NL) aimed at concerns about falls in old age as an example. After the identification of 27 BCTs within AMB-NL, an online two-round Delphi survey among 16 international experts was conducted to reach consensus on the least and most promising BCTs. The level of consensus and the level of importance of BCTs were determined. In total, 23 of the 27 (>85%) BCTs identified reached consensus. Most promising BCTs were goal setting (behavior), graded tasks and behavioral practice/rehearsal. Information about health consequences, salience of consequences and information about emotional consequences were considered least promising. These outcomes provide a first but important step in the evidence building process regarding the effectiveness of BCTs in a complex intervention.