Harbour Seals are a relatively small phocid with limited sexual dimorphism (Atlantic males/females: 153-156/140–146 cm and 75-104/67–83 kg length and mass respectively) (Härkönen and Heide-Jørgensen 1990, Lydersen and Kovacs 2010). The degree of which varies within their range. There is geographic variation in size and appearance. Harbour Seals can live for 25-30 years, reaching sexual maturity around four to five years old. 

Harbour Seals haul out and breed on a variety of habitats: rocky shores, sand banks, mud flats, rivers, and ice flows. In some haul-outs, they have become habituated, to a degree, to human presence. Foraging ranges vary markedly between regions (average extents from below 5 km to over 50 km according to Sharples et al. 2012)  and individuals with trip extents of  over 200 km and durations of  over three weeks (Lesage et al. 2004, Peterson et al. 2012). At-sea distribution is likely to vary by age class and sex (Lowry et al. 2001). Harbour Seals eat small-medium sized fish and cephalopods, with extensive geographic and seasonal variation. In freshwater, salmonoids are often the main prey. Even within limited geographic ranges they exploit a wide variety of prey (over 50 species in the UK; Wilson and Hammond 2019). They give birth to a single precocial pup which, having usually moulted in utero, can swim from birth. This allows harbour seals to take advantage of habitats for pupping that afford protection from land predators and, to some degree, human disturbance. The females continue to forage during the lactation period (Thompson et al. 1994, Bowen et al. 2001) which is around three to four weeks (Drescher 1979). The timing of pupping varies but in the Northeast Atlantic is generally between May and September (Temte 1994). Mating occurs at the end of the lactation period, with males defending at-sea territories. Moulting occurs after mating and timing varies with age and sex; juveniles moult first then adult females and finally adult males (Reder et al. 2003).  

Compared to the largely sympatric Grey Seal, they show relatively high fidelity to haul-out areas, though do occasionally move considerable distances to both haul out and breed. Dispersal appears to be highest in the pup/juvenile stage, though little is known about rates. For example, there can be significant genetic differentiation over relatively short distances even when no apparent obstacles exist (Carrol et al. 2021). However, the relationship between separation and distance is not straightforward with movements of adults between haul-outs which are over 150 km apart (Russell et al. 2016).

In the Northeast Atlantic, predators of Harbour Seal include Killer Whales (Orcinus orca), Polar Bears (Ursus maritimus), Grey Wolves (Canis lupus) and Grey Seals (Halichoerus grypus).

Harbour Seals are one of the most widespread of the pinnipeds inhabiting coastal areas of the Northern Hemisphere, from temperate to polar regions.

Within the European Red List boundary, they extend from Svalbard in the north to northern France in the south, and from Greenland in the west to Sweden (Kalmarsund) in the east. Within this spatial extent, there are some notable absences including the Faroe Islands; they are occasionally sighted in the Faroe Islands which historically hosted a subpopulation, as did the southern Baltic (as far east as Estonia; Härkönen and Isakson 2010). On a more local scale, there is little evidence for changes in distribution, even in areas which have exhibited declines (Banga et al. 2023).

Recent changes have effectively ended deliberate killing of Harbour Seals throughout most of the area. A ban on hunting is quite recent in some areas and thus the impact on populations may not have been realised (e.g. since 2010 and 2019 in Greenland and Iceland, respectively). Hunting is currently only permitted in Norway, though a Management Plan was implemented in 2010 which aims to maintain the population size (at a count of 7,000 individuals during the moult). Some hunting, under licence is permitted in Sweden. Shooting seals to fisheries has recently been banned in many countries including Norway (2019), Canada (2020), and UK (2020). In the UK such shooting was already restricted (in numbers and/or location) and it was unlikely to have contributed to any regional declines). As well as country level protections, there are specific conservation strategies on haul out specific level (e.g. Special Areas of Conservation in the European Union and the UK) to multi-country levels (e.g OSPAR). Harbour Seals are included in The Bern Convention, EU Habitats Directive, Convention on the Conservation of Migratory Species of Wild Animals (CMS; Baltic and Wadden Sea populations only), The Convention on the Protection of the Marine Environment of the Baltic Sea Area (Helsinki Convention, HELCOM) and the Agreement on the Conservation of Seals in the Wadden Sea, and their abundance and distribution are biodiversity indicators under OSPAR.

There are additional conservation measures which act on a regional or haul-out site level including Coastal Reserves in Norway which exclude commercial fishing (these have been shown to reduce harbour seal mortality), Seal Conservation Areas (Scotland), and designated haul-out sites (at which it is illegal to intentionally or recklessly harass seals; Scotland).

Further research into the population trends and threats faced by the species is recommended.