Transmission spectra are vital for probing the atmospheres of exoplanets, which motivates a deep theoretical understanding of how to interpret them. In the first part of the talk, I will review the theory of transmission spectra using both order-of-magnitude arguments and heuristic highlights of theoretical formalism. I will use a validated analytical formula for the transit radius to discuss the degeneracies inherent in interpreting transmission spectra. Specifically, transmission spectra lack an absolute normalisation, due to the a priori unknown relationship between a reference transit radius and its corresponding reference pressure. For ~1000 K atmospheres (or hotter), transmission spectra are insensitive to the pressure and temperature variations probed, with the implication that parameter exploration using a fixed grid in pressure (for the model atmosphere) may not completely elucidate the degeneracies involved. In the second part of the talk, I will discuss how the sodium and potassium lines may be used to quantify the degree of cloudiness in an exoplanetary atmosphere. Specifically, the difference in transit radius between the line core and wing can be straightforwardly calculated for a cloud-free atmosphere, and any deviation from this reference indicates the presence of clouds/hazes (of unknown composition, geometry and size distribution). I will discuss the tentative trend that hotter atmospheres appear to be less cloudy at the wavelengths probed by the alkali metal lines. I will also compare my findings to the study of Stevenson (2016), which uses the 1.4-micron spectral feature of water in the HST-WFC3 bandpass to quantify cloudiness. More generally, I will discuss how a combination of the alkali metal lines and CHEOPS and TESS photometry will enable us to efficiently screen a sample of objects for whether they possess atmospheres and if they are cloudy.