Tells are the essential building blocks of courtship, love and sex. From the very first moment that potential lovers meet, they’re looking for signs of sexual attraction and using tells to signal their interest in each other. For a loving relationship to work, two people need to be in harmony. It’s like a dance – if they don’t move together, physically and emotionally, things aren’t going to work out.     The programme begins with the simple act of kissing, showing how it’s critical for the participants to synchronise their actions. For couples who are genuinely in love synchrony comes naturally; but when people aren’t in tune with each other it’s much more difficult for them to coordinate their actions. You can also see this in the way couples walk together, hold hands, remain physically close, and are sensitive to each other’s needs and desires.     By analysing celebrity couples like Kate Winslett and Sam Mendes, Michael Jackson and Lisa Marie Presley, and Michael Douglas and Catherine Zeta Jones, we can see who’s in control, what’s happening in their relationship and where it’s heading. The tells that were in evidence when Prince Charles and Diana announced their engagement are compared with those on display when Prince Edward and Sophie announced theirs. We also compare the unconscious, giveaway signals produced during their weddings and discover that while Edward and Sophie showed all the signs of being deeply in love, Charles and Diana’s marriage was in trouble from the very start.     The programme explores the development of relationships by observing a Speedating evening and by watching what happens at a party where everybody’s a stranger. We analyse the role played by smiling, laughter and facial expressions during the speedating encounters and find that that the different flirting styles of men and women reflect the divergent evolutionary pressures on the sexes. The party provides an opportunity to observe the tells of courtship at close quarters, and to explore a wide range of tells, including the hair-flick, the eye-pop and the release pat. We discover that in a courtship dance it’s usually the woman who leads, and that women are quite happy to flirt with men even when they have no desire to take things further.     We conclude by looking at established couples and the tells they use to show everyone that they’re together. Focussing on Charles and Diana as an example, we see how the history of a couple’s relationship is written in their tells. Taking Bill Clinton as an example, we look at what the tells reveal when someone has been unfaithful to their partner. The programme shows that once you can understand the language of tells other people’s relationships become an open book. The language of tells also enables us to read our own partner, and to discover what we really feel about them.