In cardiac muscle cells, special tubes extend from the surface into the cell’s interior. Called t-tubules, these structures importantly trigger the contraction of the cell, and thus, the whole heart. In this article, state-of-the-art understanding of t-tubule structure and function are reviewed. Although we previously believed that t-tubules were rather stable structures, we now understand that they are assembled gradually in the developing heart, and that in healthy adult cells they change their shape during each heartbeat. Unfortunately, t-tubules are lost and reorganized during diseases such as heart failure, which weakens the heart’s contraction. However, it is not only the structure of t-tubules that can change during health and disease; t-tubule function also varies, due to complex arrangements of proteins in the t-tubule membranes. Although we are only beginning to understand the players that regulate t-tubule structure and function, this review highlights the important potential for these structures to be targeted by new drugs and therapies for cardiac patients.